mbuf: Add sysctl flag CTLFLAG_TUN to loader tunables

The following sysctl variables are actually loader tunables. Add sysctl
flag CTLFLAG_TUN to them so that sysctl -T will report them correctly.

1. kern.ipc.mb_use_ext_pgs
2. kern.ipc.nmbclusters
3. kern.ipc.nmbjumbop
4. kern.ipc.nmbjumbo9
5. kern.ipc.nmbjumbo16
6. kern.ipc.nmbufs

No functional change intended.

Reviewed by: kib, imp
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 6a88498e53b24d870e066c2a94910ad23dd355eb)
(cherry picked from commit b1456fd6209474410c031e5e82455588e2b656c0)