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>