epoll: use RCU to protect wakeup_source in epitem

This prevents wakeup_source destruction when a user hits the item with
EPOLL_CTL_MOD while ep_poll_callback is running.

Tested with CONFIG_SPARSE_RCU_POINTER=y and "make fs/eventpoll.o C=2"

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: NeilBrown <neilb@suse.de>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>