xfs: fix XFS_IOC_FREE_EOFBLOCKS definition

It uses a kernel internal structure in it's definition rather than
the user visible structure that is passed to the ioctl.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>