xfs: refactor xfs_inode_verify_forks

The split between xfs_inode_verify_forks and the two helpers
implementing the actual functionality is a little strange.  Reshuffle
it so that xfs_inode_verify_forks verifies if the data and attr forks
are actually in local format and only call the low-level helpers if
that is the case.  Handle the actual error reporting in the low-level
handlers to streamline the caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>