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.

[[e-mail]: fix printk warning]
Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: OGAWA Hirofumi <[e-mail]>
Reported-by: Anatoly Trosinenko <[e-mail]>
Tested-by: Anatoly Trosinenko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fat/inode.c

···
 	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);