jails: delegate checking PRIV_PROC_MEM_WRITE to priv_check_cred()

PR: 285811
Fixes: 4a5fa1086184f7450f63d4a8e403b16f40a78fce
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D49682