docs: filesystems: convert romfs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/d2cc83[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/index.rst

···
    qnx6
    ramfs-rootfs-initramfs
    relay
+   romfs
    virtiofs
    vfat

Documentation/filesystems/romfs.txt Documentation/filesystems/romfs.rst

···
-ROMFS - ROM FILE SYSTEM
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+ROMFS - ROM File System
+=======================
 
 This is a quite dumb, read only filesystem, mainly for initial RAM
 disks of installation disks.  It has grown up by the need of having
···
 bytes.  This is quite rare however, since most file names are longer
 than 3 bytes, and shorter than 15 bytes.
 
-The layout of the filesystem is the following:
+The layout of the filesystem is the following::
 
-offset	    content
+ offset	    content
 
 	+---+---+---+---+
   0	| - | r | o | m |  \
···
 reliable, it does not require any tables, and it is very simple.
 
 The following bytes are now part of the file system; each file header
-must begin on a 16 byte boundary.
+must begin on a 16 byte boundary::
 
-offset	    content
+ offset	    content
 
      	+---+---+---+---+
   0	| next filehdr|X|	The offset of the next file header
···
 intended use.  The mapping of the 8 possible values to file types is
 the following:
 
+==	=============== ============================================
 	  mapping		spec.info means
+==	=============== ============================================
  0	hard link	link destination [file header]
  1	directory	first file's header
  2	regular file	unused, must be zero [MBZ]
···
  5	char device		    - " -
  6	socket		unused, MBZ
  7	fifo		unused, MBZ
+==	=============== ============================================
 
 Note that hard links are specifically marked in this filesystem, but
 they will behave as you can expect (i.e. share the inode number).
···
 Pending issues:
 
 - Permissions and owner information are pretty essential features of a
-Un*x like system, but romfs does not provide the full possibilities.
-I have never found this limiting, but others might.
+  Un*x like system, but romfs does not provide the full possibilities.
+  I have never found this limiting, but others might.
 
 - The file system is read only, so it can be very small, but in case
-one would want to write _anything_ to a file system, he still needs
-a writable file system, thus negating the size advantages.  Possible
-solutions: implement write access as a compile-time option, or a new,
-similarly small writable filesystem for RAM disks.
+  one would want to write _anything_ to a file system, he still needs
+  a writable file system, thus negating the size advantages.  Possible
+  solutions: implement write access as a compile-time option, or a new,
+  similarly small writable filesystem for RAM disks.
 
 - Since the files are only required to have alignment on a 16 byte
-boundary, it is currently possibly suboptimal to read or execute files
-from the filesystem.  It might be resolved by reordering file data to
-have most of it (i.e. except the start and the end) laying at "natural"
-boundaries, thus it would be possible to directly map a big portion of
-the file contents to the mm subsystem.
+  boundary, it is currently possibly suboptimal to read or execute files
+  from the filesystem.  It might be resolved by reordering file data to
+  have most of it (i.e. except the start and the end) laying at "natural"
+  boundaries, thus it would be possible to directly map a big portion of
+  the file contents to the mm subsystem.
 
 - Compression might be an useful feature, but memory is quite a
-limiting factor in my eyes.
+  limiting factor in my eyes.
 
 - Where it is used?
 
···
 
 
 Have fun,
+
 Janos Farkas <[e-mail]>