fat: use bdev_nr_sectors instead of open coding it

Use the proper helper to read the block device size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211018101130.1838532-15-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/fat/inode.c

@@ -1536,14 +1536,11 @@ static int fat_read_static_bpb(struct super_block *sb,
 	struct fat_bios_param_block *bpb)
 {
 	static const char *notdos1x = "This doesn't look like a DOS 1.x volume";
-
+	sector_t bd_sects = bdev_nr_sectors(sb->s_bdev);
 	struct fat_floppy_defaults *fdefaults = NULL;
 	int error = -EINVAL;
-	sector_t bd_sects;
 	unsigned i;
 
-	bd_sects = i_size_read(sb->s_bdev->bd_inode) / SECTOR_SIZE;
-
 	/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
 	if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90) {
 		if (!silent)