UPSTREAM: mm/memfd: make F_SEAL_FUTURE_WRITE seal more robust

A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
where we don't need to modify core VFS structures to get the same
behavior of the seal. This solves several side-effects pointed out by
Andy [2].

[1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
[2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/



Suggested-by: Andy Lutomirski <luto@kernel.org>
Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")
Change-id: I5d2414cfcf8ac42d3632d0b0dc960c742d490e2f
Verified with test program at: https://lore.kernel.org/patchwork/patch/1008117/
Backport link: https://lore.kernel.org/patchwork/patch/1014892/


Bug: 113362644
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>