netgraph/ng_bridge: Add counters for the first link, too

For broadcast, multicast and unknown unicast, the replication loop
sends a copy of the packet to each link, beside the first one. This
special path is handled later, but the counters are not updated.
Factor out the common send and count actions as a function.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28537