umtx: shm: 'ushm_refcnt > 0' => 'ushm_refcnt != 0'
'ushm_refcnt' is unsigned. Don't leave the impression it isn't.
No functional change (intended).
Reviewed by: kib
Approved by: emaste (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46126