fs/epoll: remove unnecessary wakeups of nested epoll

Take the case where we have:

t0
| (ew)
e0
| (et)
e1
| (lt)
s0

t0: thread 0
e0: epoll fd 0
e1: epoll fd 1
s0: socket fd 0
ew: epoll_wait
et: edge-trigger
lt: level-trigger

We remove unnecessary wakeups to prevent the nested epoll that working in edge-
triggered mode to waking up continuously.

Test code:
#include <unistd.h>
#include <sys/epoll.h>
#include <sys/socket.h>

int main(int argc, char *argv[])
{
int sfd[2];
int efd[2];
struct epoll_event e;

if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0)
goto out;

efd[0] = epoll_create(1);
if (efd[0] < 0)
goto out;

efd[1] = epoll_create(1);
if (efd[1] < 0)
goto out;

e.events = EPOLLIN;
if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
goto out;

e.events = EPOLLIN | EPOLLET;
if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
goto out;

if (write(sfd[1], "w", 1) != 1)
goto out;

if (epoll_wait(efd[0], &e, 1, 0) != 1)
goto out;

if (epoll_wait(efd[0], &e, 1, 0) != 0)
goto out;

close(efd[0]);
close(efd[1]);
close(sfd[0]);
close(sfd[1]);

return 0;

out:
return -1;
}

More tests:
https://github.com/heiher/epoll-wakeup

Link: http://lkml.kernel.org/r/20191009060516.3577-1-r@hev.cc
Signed-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Eric Wong <e@80x24.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>