bridge: Add support for emulated netmap mode

if_bridge receives packets via a special interface, if_bridge_input,
rather than by if_input. Thus, netmap's usual hooking of ifnet routines
does not work as expected. Instead, modify bridge_input() to pass
packets directly to netmap when it is enabled. This applies to both
locally delivered packets and forwarded packets.

When a netmap application transmits a packet by writing it to the host
TX ring, the mbuf chain is passed to if_input, which ordinarily points
to ether_input(). However, when transmitting via if_bridge,
bridge_input() needs to see the packet again in order to decide whether
to deliver or forward. Thus, introduce a new protocol flag,
M_BRIDGE_INJECT, which 1) causes the packet to be passed to
bridge_input() again after Ethernet processing, and 2) avoids passing
the packet back to netmap. The source MAC address of the packet is used
to determine the original "receiving" interface.

Reviewed by: vmaffione
MFC after: 2 months
Sponsored by: Zenarmor
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38066