tmpfs: turn vfs.tmpfs.memory_percent into CTLFLAG_RWTUN

So sysctl is not the only way to set the value of this knob, this can be
set by loader tunables as well.

Sponsored by: Juniper Networks, Inc.
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49763