[XFS] cleanup fid types mess

Currently XFs has three different fid types: struct fid, struct xfs_fid
and struct xfs_fid2 with hte latter two beeing identicaly and the first
one beeing the same size but an unstructured array with the same size.

This patch consolidates all this to alway uuse struct xfs_fid.

This patch is required for an upcoming patch series from me that revamps
the nfs exporting code and introduces a Linux-wide struct fid.

SGI-PV: 970336
SGI-Modid: xfs-linux-melb:xfs-kern:29651a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>