fat: remove FAT_FIRST_ENT macro

The comment edited in this patch was the only reference to the
FAT_FIRST_ENT macro, which is not used anymore. Moreover, the commented
line of code does not compile with the current code.

Since the FAT_FIRST_ENT macro checks the FAT variant in a way that the
patch series changes, I removed it, and instead wrote a clear
explanation of what was checked.

I verified that the changed comment is correct according to Microsoft
FAT spec, search for "BPB_Media" in the following references:

1. Microsoft FAT specification 2005
(http://read.pudn.com/downloads77/ebook/294884/FAT32%20Spec%20%28SDA%20Contribution%29.pdf).
Search for 'volume label'.
2. Microsoft Extensible Firmware Initiative, FAT32 File System Specification
(https://staff.washington.edu/dittrich/misc/fatgen103.pdf).
Search for 'volume label'.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Carmeli Tamir <[e-mail]>
Acked-by: OGAWA Hirofumi <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Cc: Bart Van Assche <[e-mail]>
Cc: Johannes Thumshirn <[e-mail]>
Cc: Martin K. Petersen <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fat/inode.c

···
 	fat_ent_access_init(sb);
 
 	/*
-	 * The low byte of FAT's first entry must have same value with
-	 * media-field.  But in real world, too many devices is
-	 * writing wrong value.  So, removed that validity check.
+	 * The low byte of the first FAT entry must have the same value as
+	 * the media field of the boot sector. But in real world, too many
+	 * devices are writing wrong values. So, removed that validity check.
 	 *
-	 * if (FAT_FIRST_ENT(sb, media) != first)
+	 * The removed check compared the first FAT entry to a value dependent
+	 * on the media field like this:
+	 * == (0x0F00 | media), for FAT12
+	 * == (0XFF00 | media), for FAT16
+	 * == (0x0FFFFF | media), for FAT32
 	 */
 
 	error = -EINVAL;

include/uapi/linux/msdos_fs.h

···
 #define MSDOS_DOT	".          "	/* ".", padded to MSDOS_NAME chars */
 #define MSDOS_DOTDOT	"..         "	/* "..", padded to MSDOS_NAME chars */
 
-#define FAT_FIRST_ENT(s, x)	((MSDOS_SB(s)->fat_bits == 32 ? 0x0FFFFF00 : \
-	MSDOS_SB(s)->fat_bits == 16 ? 0xFF00 : 0xF00) | (x))
-
 /* start of data cluster's entry (number of reserved clusters) */
 #define FAT_START_ENT	2