There is measurable performance impact in some synthetic tests due to
commit 6d390e4b (locks: fix a potential use-after-free problem when
wakeup a waiter). Fix the race condition instead by clearing the
fl_blocker pointer after the wake_up, using explicit acquire/release
semantics.

This does mean that we can no longer use the clearing of fl_blocker as
the wait condition, so switch the waiters over to checking whether the
fl_blocked_member list_head is empty.
Reviewed-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: NNeilBrown <neilb@suse.de>
Fixes: 6d390e4b (locks: fix a potential use-after-free problem when wakeup a waiter)
Signed-off-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>