Fix: fencer: Avoid double source remove of op_timer_total

remote_op_timeout() returns G_SOURCE_REMOVE, which tells GLib to remove
the source from the main loop after returning. Currently this function
is used as the callback only when creating op->op_timer_total.

If we don't set op->op_timer_total to 0 before returning from
remote_op_timeout(), then we can get an assertion and core dump from
GLib when the op's timers are being cleared (either during op
finalization or during fencer shutdown). This is because
clear_remote_op_timers() sees that op->op_timer_total != 0 and tries to
remove the source, but the source has already been removed.

Note that we're already (correctly) zeroing op->op_timer_one and
op->query_timeout as appropriate in their respective callback functions.

Fortunately, GLib doesn't care whether the source has already been
removed before we return G_SOURCE_REMOVE from a callback. So it's safe
to call finalize_op() (which removes all the op's timer sources) from
within a callback.

Fixes RHBZ#2166967

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>