umtx: Add a helper for unlocked umtxq_busy() calls

This seems like a natural complement to umtxq_unbusy_unlocked(). No
functional change intended.

Reviewed by: olce, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49124

(cherry picked from commit b01495caac2eca73463f4a889936a19e4c1c5909)