Fix linprocfs(4) /proc/self/mem semantics to more closely match Linux.
Steam's Anti-Cheat might depend on it.

PR: 248223
Analyzed by: Alex S <iwtcex@gmail.com>
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26816