xfs: check inode core when scrubbing metadata files

Metadata files (e.g. realtime bitmaps and quota files) do not show up in
the bulkstat output, which means that scrub-by-handle does not work;
they can only be checked through a specific scrub type.  Therefore, each
scrub type calls xchk_metadata_inode_forks to check the metadata for
whatever's in the file.

Unfortunately, that function doesn't actually check the inode record
itself.  Refactor the function a bit to make that happen.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>