[IPV4] UDP,UDPLITE: Sparse: {__udp4_lib,udp,udplite}_err() are of void.

Fix following sparse warnings:
| net/ipv4/udp.c:421:2: warning: returning void-valued expression
| net/ipv4/udplite.c:38:2: warning: returning void-valued expression

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>