memfd_create.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/T/#t

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
1 file changed