ng_bridge: Use M_NOWAIT when allocating memory in the newhook routine

newhook can be invoked by ngthread, which runs in a network epoch
section and is thus not permitted to perform M_WAITOK allocations.

Reported by: Jenkins
Reviewed by: donner, afedorov
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e0e3ded78a5d0859f3520c541726b815897ba7b0)