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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31486