MFC r368588:

pf: Allow net.pf.request_maxcount to be set from loader.conf

Mark request_maxcount as RWTUN so we can set it both at runtime and from
loader.conf. This avoids users getting caught out by the change from tunable to
run time configuration.

Suggested by: Franco Fichtner
Approved by: so
Security: FreeBSD-EN-21:06.pf

(cherry picked from commit 08d13750ebdae45bcdb73d52665b823e9ba93db1)