net: move zerocopy_sg_from_iovec() to net/core/datagram.c

To let it be reused and reduce code duplication. Also document this function.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/macvtap.c
Simple merge

diff --cc drivers/net/tun.c
Simple merge

diff --cc include/linux/skbuff.h
Simple merge

diff --cc net/core/datagram.c
Simple merge