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

(cherry picked from commit c75a18905e308f69b01f19c3d7d613883a008e79)
(cherry picked from commit 4938f554469bc81783998143968ec3eba2a43def)