Fix interaction between largepages and seals/writes.

On write with SHM_GROW_ON_WRITE, use proper truncate.
Do not allow to grow largepage shm if F_SEAL_GROW is set. Note that
shrinks are not supported at all due to unmanaged mappings.
Call to vm_pager_update_writecount() is only valid for swap objects,
skip it for unmanaged largepages.
Largepages cannot support write sealing.
Do not writecnt largepage mappings.

Reported by: kevans
Reviewed by: kevans, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26394