udp: Do not recursively enter net epoch

The only caller udp_send() has already entered net epoch before invoking
udp_v4mapped_pktinfo().

No functional change intended.

This partially reverts commit d74b7baeb0d4 (ifnet_byindex() actually
requires network epoch).

Reviewed by: ae, glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49227