udp: Fix port-specific ptp/p2p_dst_ipv4 configuration.

If different ports are configured with a different ptp_dst_ipv4 or
p2p_dst_ipv4 address, only the last port in the configuration works
correctly. This is caused by a global variable holding the
destination address for all ports using the udp transport.

Move the address to the udp structure to avoid the conflict between
different ports, same as when port-specific scope in udp6 was fixed
in commit a48666bee3dd ("udp6: Make mc6_addr transport-local").

Fixes: 8a26c94cc88e ("udp+udp6: Make IP addresses configurable.")
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>