xfs: rationalize xfs_inobt_lookup*

Currenly we have a xfs_inobt_lookup* variant for each comparism direction,
and all these get all three fields of the inobt records passed, while the
common case is just looking for the inode number and we have only marginally
more callers than xfs_inobt_lookup* variants.

So opencode a direct call to xfs_btree_lookup for the single case where we
need all fields, and replace xfs_inobt_lookup* with a xfs_inobt_looku that
just takes the inode number and the direction for all other callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>