net: udp4: move GSO functions to udp_offload

Similarly to TCP offloading and UDPv6 offloading, move all related
UDPv4 functions to udp_offload.c to make things more explicit. Also,
by this, we can make those functions static.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/udp.h
Simple merge

diff --cc net/ipv4/Makefile
Simple merge

diff --cc net/ipv4/af_inet.c
Simple merge

diff --cc net/ipv4/udp.c
Simple merge

diff --cc net/ipv4/udp_offload.c
Simple merge