fat: use fat_fs_error() instead of BUG_ON() in __fat_get_block()

If file size and FAT cluster chain is not matched (corrupted image), we
can hit BUG_ON(!phys) in __fat_get_block().

So, use fat_fs_error() instead.

[hirofumi@mail.parknet.co.jp: fix printk warning]
  Link: http://lkml.kernel.org/r/87po12aq5p.fsf@mail.parknet.co.jp
Link: http://lkml.kernel.org/r/874lilcu67.fsf@mail.parknet.co.jp
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Tested-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/inode.c

@@ -158,8 +158,14 @@ static inline int __fat_get_block(struct inode *inode, sector_t iblock,
 	err = fat_bmap(inode, iblock, &phys, &mapped_blocks, create, false);
 	if (err)
 		return err;
+	if (!phys) {
+		fat_fs_error(sb,
+			     "invalid FAT chain (i_pos %lld, last_block %llu)",
+			     MSDOS_I(inode)->i_pos,
+			     (unsigned long long)last_block);
+		return -EIO;
+	}
 
-	BUG_ON(!phys);
 	BUG_ON(*max_blocks != mapped_blocks);
 	set_buffer_new(bh_result);
 	map_bh(bh_result, sb, phys);