xfs/216: pass -t xfs to _mount

xfs/216: pass -t xfs to _mount

Without this, mount probes different file system type, which ended up
exposing kernel bugs in blocksize > PAGE_SIZE bugs in tons of
legacy file systems.  Bypass that by forcing XFS and making the
mount a little more efficient.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>