[NET]: Use sockfd_lookup_light in the rest of the net/socket.c

Some time ago a sockfd_lookup_light was introduced and
most of the socket.c file was patched to use it. However
two routines were left - sys_sendto and sys_recvfrom.

Patch them as well, since this helper does exactly what
these two need.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>