selftests/memfd: add tests for F_SEAL_FUTURE_WRITE seal

Add tests to verify sealing memfds with the F_SEAL_FUTURE_WRITE works as
expected.

Link: http://lkml.kernel.org/r/20190112203816.85534-3-joel@joelfernandes.org


Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Shuah Khan <shuah@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>