lockf: hide unavailable error with -n -s

The error message is expected, allow -s to suppress just that one since
it would loosely fall under the definition of "failure to acquire the
lock" described in the manpage for the -s option.

Reviewed by: 0mp, allanjude
Feedback from: des
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D42711