xfs: reverse the check on XFS_IOC_ZERO_RANGE

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: Andrew Dahl <adahl@sgi.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>