udf: Use IS_ERR when loading metadata mirror file entry

Currently when udf_get_pblock_meta25() fails to map a block using the
primary metadata file, it will attempt to load the mirror file entry by
calling udf_find_metadata_inode_efe().  That function will return a ERR_PTR
if it fails, but the return value is only checked against NULL.  Test the
return value using IS_ERR() and change it to NULL if needed.
Signed-off-by: Alden Tondettar <alden.tondettar@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/partition.c

@@ -326,6 +326,8 @@ uint32_t udf_get_pblock_meta25(struct super_block *sb, uint32_t block,
 		if (!(mdata->s_flags & MF_MIRROR_FE_LOADED)) {
 			mdata->s_mirror_fe = udf_find_metadata_inode_efe(sb,
 				mdata->s_mirror_file_loc, map->s_partition_num);
+			if (IS_ERR(mdata->s_mirror_fe))
+				mdata->s_mirror_fe = NULL;
 			mdata->s_flags |= MF_MIRROR_FE_LOADED;
 		}