[XFS] kill the vfs_fsid and vfs_altfsid members in struct bhv_vfs

vfs_altfsid was just a pointer to mp->m_fixedfsid so we can trivially
replace it with the latter. vfs_fsid also was identical to m_fixedfsid
through rather obfuscated ways so we can kill it as well and simply its
only user.

SGI-PV: 969608
SGI-Modid: xfs-linux-melb:xfs-kern:29506a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>