include/uapi/linux/msdos_fs.h: use MSDOS_NAME for volume label size

The FAT file system volume label file stored in the root directory
should match the volume label field in the FAT boot sector.  As
consequence, the max length of these fields ought to be the same.  This
patch replaces the magic '11' usef in the struct fat_boot_sector with
MSDOS_NAME, which is used in struct msdos_dir_entry.

Please check 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'.
3. User space code that creates FAT filesystem
sometimes uses MSDOS_NAME for the label, sometimes not.
Search for 'if (memcmp(label, NO_NAME, MSDOS_NAME))'.
I consider to make the same patch there as well.
https://github.com/dosfstools/dosfstools/blob/master/src/mkfs.fat.c

Link: http://lkml.kernel.org/r/1543096879-82837-1-git-send-email-carmeli.tamir@gmail.com
Signed-off-by: Carmeli Tamir <carmeli.tamir@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/uapi/linux/msdos_fs.h

@@ -135,7 +135,7 @@ struct fat_boot_sector {
 						   for mount state. */
 			__u8	signature;  /* extended boot signature */
 			__u8	vol_id[4];	/* volume ID */
-			__u8	vol_label[11];	/* volume label */
+			__u8	vol_label[MSDOS_NAME];	/* volume label */
 			__u8	fs_type[8];		/* file system type */
 			/* other fields are not added here */
 		} fat16;
@@ -158,7 +158,7 @@ struct fat_boot_sector {
 						   for mount state. */
 			__u8	signature;  /* extended boot signature */
 			__u8	vol_id[4];	/* volume ID */
-			__u8	vol_label[11];	/* volume label */
+			__u8	vol_label[MSDOS_NAME];	/* volume label */
 			__u8	fs_type[8];		/* file system type */
 			/* other fields are not added here */
 		} fat32;