proc_rwmem check PRIV_PROC_MEM_WRITE when writing

This will fail when mac_veriexec is enforced.

Move the check from procfs_doprocmem to proc_rwmem to ensure all
cases are covered.

Reviewed by: olce, markj
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D47484