procfs require PRIV_PROC_MEM_WRITE to write mem

Add a priv_check for PRIV_PROC_MEM_WRITE which will be blocked
by mac_veriexec if being enforced, unless the process has a maclabel
to grant priv.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D46692