pf: Initialize the table entry zone limit at initialization time

pf: Initialize the table entry zone limit at initialization time

The limit may later be updated by the "set limit" directive in pf.conf.
UMA does not permit a limit to be set on a zone after any items have
been allocated from a zone.

Other UMA zones used by pf do not appear to be susceptible to this
problem: they either set a limit at zone creation time or never set one
at all.

PR:		260406
Reviewed by:	kp
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D34713