epoll: remove the on_list check for 'struct epitem'

epoll: remove the on_list check for 'struct epitem'

By removing the 'int on_list' field from 'struct epitem', we avoid hitting
the BUILD_BUG_ON() for 'struct epitem' being larger than 128 bytes.

In file included from include/linux/init.h:4:0,
                 from fs/eventpoll.c:14:
fs/eventpoll.c: In function 'eventpoll_init':
include/linux/compiler.h:321:20: error: call to '__compiletime_assert_2137' declared with attribute error: BUILD_BUG_ON failed: sizeof(void *) <= 8 && sizeof(struct epitem) > 128
    prefix ## suffix();    \

The check to make sure that the 'struct epitem' was actually linked via
epi->fllink was added to avoid having the list removal primitives called
twice for the same 'struct epitem'.  However, the double call possibility
was removed by 'Subject: epoll: optimize EPOLL_CTL_DEL using rcu'.  There,
the call to 'list_del_init()' in eventpoll_release_file() was removed (we
now rely on the list delete happening entirely in 'ep_remove()', which is
called from eventpoll_release_file()).

There is also the question as to whether multiple ep_remove() calls could
happen concurrently.  This can not happen since EPOLL_CTL_DEL can't race
with eventpoll_release_file() or ep_free() - it has to do an fget() to
proceed.  Further, eventpoll_release_file() can not race with ep_free(),
since they both acquire the 'epmutex'.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Nathan Zimmer <nzimmer@sgi.com>
Cc: Eric Wong <normalperson@yhbt.net>
Cc: Nelson Elhage <nelhage@nelhage.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>