don't reallocate sxp variable passed into xfs_swapext

fixes kernel.org bugzilla 12538, xfs_fsr fails on 2.6.29-rc kernels

Regression caused by 743bb465

This was an embarrasing mistake, reallocating the sxp pointer passed
in from the main ioctl switch.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reported-by: Paul Martin <pm@debian.org>
Tested-by: Paul Martin <pm@debian.org>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>