selftests/memfd: modify tests for F_SEAL_FUTURE_WRITE seal

selftests/memfd: modify tests for F_SEAL_FUTURE_WRITE seal

Modify the tests for F_SEAL_FUTURE_WRITE based on the changes
introduced in previous patch.

Also add a test to make sure the reopen issue pointed by Jann Horn [1]
is fixed.

[1] https://lore.kernel.org/lkml/CAG48ez1h=v-JYnDw81HaYJzOfrNhwYksxmc2r=cJvdQVgYM+NA@mail.gmail.com/

Link: http://lkml.kernel.org/r/20181120052137.74317-2-joel@joelfernandes.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Jann Horn <jannh@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Daniel Colascione <dancol@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: John Reck <jreck@google.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Lei Yang <Lei.Yang@windriver.com>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>