- 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)