[XFS] Cleanup various fid related bits:

- merge xfs_fid2 into it's only caller xfs_dm_inode_to_fh.
- remove xfs_vget and opencode it in the two callers, simplifying
  both of them by avoiding the awkward calling convetion.
- assign directly to the dm_fid_t members in various places in the
  dmapi code instead of casting them to xfs_fid_t first (which
  is identical to dm_fid_t)

SGI-PV: 974747
SGI-Modid: xfs-linux-melb:xfs-kern:30258a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Vlad Apostolov <vapo@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>