pfsync: hold b_mtx for callout_stop(pd_tmo)

The pd_tmo callout has an associated mutex, which we must hold while
calling callout_stop().

Reported by: markj
Reviewed by: markj
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39223

(cherry picked from commit 01194da28a2123a2aa09808319f152fa115f60f1)