epoll: use refcount to reduce ep_mutex contention
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2044953 Tested: Regression test with xfstests
Backport the eventpoll optimization achieved by using a refcount and a couple of extra patches as dependencies.
Signed-off-by: Carlos Maiolino cmaiolino@redhat.com