selftests: add epoll selftests

This adds the promised selftest for epoll.  It will verify the wakeups
of epoll.  Including leaf and nested mode, epoll_wait() and poll() and
multi-threads.

Link: http://lkml.kernel.org/r/20191009121518.4027-1-r@hev.ccSigned-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/Makefile

@@ -13,6 +13,7 @@ TARGETS += efivarfs
 TARGETS += exec
 TARGETS += filesystems
 TARGETS += filesystems/binderfs
+TARGETS += filesystems/epoll
 TARGETS += firmware
 TARGETS += ftrace
 TARGETS += futex

tools/testing/selftests/filesystems/epoll/.gitignore

+epoll_wakeup_test

tools/testing/selftests/filesystems/epoll/Makefile

+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += -I../../../../../usr/include/
+LDFLAGS += -lpthread
+TEST_GEN_PROGS := epoll_wakeup_test
+
+include ../../lib.mk