xfs: remove xfs_ifork_ops

xfs_ifork_ops add up to two indirect calls per inode read and flush,
despite just having a single instance in the kernel.  In xfsprogs
phase6 in xfs_repair overrides the verify_dir method to deal with inodes
that do not have a valid parent, but that can be fixed pretty easily
by ensuring they always have a valid looking parent.

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