We don't have a way to flush a timer that's executing the callback, and
this is simple and limited enough in scope that we can just use the lock
instead.

Needed for the next patch that adds direct wakeups from the allocator to
copygc, where we're now more frequently calling io_timer_del() on an
expiring timer.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>