selftests: add eventfd selftests

This adds the promised selftest for eventfd.  It will verify the flags of
eventfd2, including EFD_CLOEXEC, EFD_NONBLOCK and EFD_SEMAPHORE.

Link: https://lkml.kernel.org/r/tencent_3C9A298878D22B5D8F79DC2FEE99BB4A8F05@qq.com


Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Pengfei Xu <pengfei.xu@intel.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Andrei Vagin <avagin@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>