sysfs-namespaces: add a high-level Documentation file

The first three paragraphs are almost verbatim taken from Eric's
commit message on the patch introducing network ns tags. The next
two paragraphs I wrote to be a brief high level overview. The last
section is taken from the commit message on "Implement sysfs tagged
directory support", but updated. Hopefully correctly.

Signed-off-by: Serge E. Hallyn <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/filesystems/sysfs-tagging.txt

···
+Sysfs tagging
+-------------
+
+(Taken almost verbatim from Eric Biederman's netns tagging patch
+commit msg)
+
+The problem.  Network devices show up in sysfs and with the network
+namespace active multiple devices with the same name can show up in
+the same directory, ouch!
+
+To avoid that problem and allow existing applications in network
+namespaces to see the same interface that is currently presented in
+sysfs, sysfs now has tagging directory support.
+
+By using the network namespace pointers as tags to separate out the
+the sysfs directory entries we ensure that we don't have conflicts
+in the directories and applications only see a limited set of
+the network devices.
+
+Each sysfs directory entry may be tagged with zero or one
+namespaces.  A sysfs_dirent is augmented with a void *s_ns.  If a
+directory entry is tagged, then sysfs_dirent->s_flags will have a
+flag between KOBJ_NS_TYPE_NONE and KOBJ_NS_TYPES, and s_ns will
+point to the namespace to which it belongs.
+
+Each sysfs superblock's sysfs_super_info contains an array void
+*ns[KOBJ_NS_TYPES].  When a a task in a tagging namespace
+kobj_nstype first mounts sysfs, a new superblock is created.  It
+will be differentiated from other sysfs mounts by having its
+s_fs_info->ns[kobj_nstype] set to the new namespace.  Note that
+through bind mounting and mounts propagation, a task can easily view
+the contents of other namespaces' sysfs mounts.  Therefore, when a
+namespace exits, it will call kobj_ns_exit() to invalidate any
+sysfs_dirent->s_ns pointers pointing to it.
+
+Users of this interface:
+- define a type in the kobj_ns_type enumeration.
+- call kobj_ns_type_register() with its kobj_ns_type_operations which has
+  - current_ns() which returns current's namespace
+  - netlink_ns() which returns a socket's namespace
+  - initial_ns() which returns the initial namesapce
+- call kobj_ns_exit() when an individual tag is no longer valid