[XFS] Add compat handlers for swapext ioctl

The big hitter here was the bstat field, which contains
different sized time_t on 32 vs. 64 bit.  Add a copyin
function to translate the 32-bit arg to 64-bit, and
call the swapext ioctl helper.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>