memfd: add test for COW on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

In this test, the parent and child both have writable private mappings.
The test shows that without the patch in this series, the parent and
child shared the same memory which is incorrect.  In other words, COW
needs to be triggered so any writes to child's copy stays local to the
child.

Link: http://lkml.kernel.org/r/20191107195355.80608-2-joel@joelfernandes.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>