xfs: remove the NULL fork handling in xfs_bmapi_read

Now that we fully verify the inode forks before they are added to the
inode cache, the crash reported in

  https://bugzilla.kernel.org/show_bug.cgi?id=204031



can't happen anymore, as we'll never let an inode that has inconsistent
nextents counts vs the presence of an in-core attr fork leak into the
inactivate code path.  So remove the work around to try to handle the
case, and just return an error and warn if the fork is not present.

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>