According to the documentation, on Linux, cancel_delayed_work() does not
do drain (flush_workqueue() in Linux terms) but instead returns true if
the work was removed before it is run, or false otherwise.

Simulate this by removing the taskqueue_drain() and return the value
derived from taskqueue_cancel()'s return value.

This would solve a witness warning caused by calling taskqueue_drain()
with a non-sleepable lock held, like:

taskqueue_drain with the following non-sleepable locks held:
exclusive rw lle (lle) r = 0 (0xfffffe001450b410) locked @
/usr/src/sys/netinet/in.c:1484
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffff848d4f7690
kdb_backtrace() at kdb_backtrace+0x39/frame 0xffffff848d4f7740
witness_warn() at witness_warn+0x4a8/frame 0xffffff848d4f7800
taskqueue_drain() at taskqueue_drain+0x3a/frame 0xffffff848d4f7840
set_timeout() at set_timeout+0x4a/frame 0xffffff848d4f7860
netevent_callback() at netevent_callback+0x16/frame 0xffffff848d4f7870
arpintr() at arpintr+0x9b5/frame 0xffffff848d4f7930

This do not affect kernel without OFED compiled in.

Reported by:	Garrett Cooper <yaneurabeya gmail com>
		(who also tested an earlier version of this patch,
		but bugs are mine)
MFC after:	2 weeks