udp: Prefer memcpy() over bcopy()

The variable b[] is on the stack, thus cannot overlap with ipov, which
points to the heap area, so prefer memcpy() over memmove(), aka bcopy().

No functional change intended.

Reviewed by: cc, rrs, cy, transport, network
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47713

(cherry picked from commit 949190c5afbc677c53b5d96354b52256923a11d1)