bsnmp: Fix incorrect use of libpfctl

When we introduced libpfctl in bnsmpd this was done incorrectly, and a
GETRULE call was replaced by pfctl_add_rule().
Change it to pfctl_get_rule().

Reviewed by: donner
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 67cef6d411cef0c3dfcf768ff890b4f97cf513b6)