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