In do_lock_pi(), do not return prematurely.

If umtxq_check_susp() indicates an exit, we should clean the resources
before returning. Do it by breaking out of the loop and relying on
post-loop cleanup.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 12 days
Differential revision: https://reviews.freebsd.org/D20949