In do_lock_pi(), do not override error from umtxq_sleep_pi() when
doing suspend check. This restores the pre-r251684 behaviour, to
retry once after the signal is detected.

PR: kern/192918
Submitted by: Elliott Rabe, Dell Inc., Eric van Gyzen <eric@vangyzen.net>
Obtained from: Dell Inc.
MFC after: 1 week