UPSTREAM: 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.

Cc: dancol@google.com
Cc: minchan@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Shuah Khan <shuah@kernel.org>
Change-Id: I42a0ceb1f584a5444ec755f8c76899499bcbaddd