selftests/filesystems: expand epoll with epoll_pwait2

selftests/filesystems: expand epoll with epoll_pwait2

Code coverage for the epoll_pwait2 syscall.

epoll62: Repeat basic test epoll1, but exercising the new syscall.
epoll63: Pass a timespec and exercise the timeout wakeup path.

Link: https://lkml.kernel.org/r/20201121144401.3727659-5-willemdebruijn.kernel@gmail.com
Signed-off-by: Willem de Bruijn <willemb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>