[XFS] move XFS_IOC_GETVERSION to main multiplexer

Avoids doing an unnecessary inode to vnode conversion and avoids a memory
allocation.

SGI-PV: 904196
SGI-Modid: xfs-linux-melb:xfs-kern:26492a

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>