MFC r276534:

MFC r276534:
The "vt_suspend_flush_timer()" function is sometimes called locked
which prevents us from doing a "callout_drain()" call. The callout in
question has a lock associated with it and we are not freeing the
callout. That means we can use the "callout_stop()" function to
atomically stop the callback iff the "callout_stop()" function is
called locked. This patch applies proper locking to "callout_stop()"
and replaces a "callout_drain()" with a "callout_stop()".

git-svn-id: svn://svn.freebsd.org/base/stable/10@276972 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f