lockf: Test that lockf does not spin on fd

Approved by: re (cperciva)
PR: 294832
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D56723

(cherry picked from commit 0733afdb4dad58cfe7ad7f5994e26bf148d78199)
(cherry picked from commit 8e5f92b6b22c82af1192575781788eb2c8fed448)