Reversing the check on XFS_IOC_ZERO_RANGE.

Range should be zeroed if the start is less than or equal to the end.
Signed-off-by: NAndrew Dahl <adahl@sgi.com>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>