selftests/futex: Fix futex_cmp_requeue_pi() error handling

An earlier (pre-kernel-integration) refactoring of this code mistakenly
replaced the error condition, <, with a >. Use < to detect an error as
opposed to a successful requeue or signal race.

Reported-by: David Binderman <dcb314@hotmail.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>