[IPV6]: Sparse: Reuse previous delaration where appropriate.

| net/ipv6/ipv6_sockglue.c:162:16: warning: symbol 'net' shadows an earlier one
| net/ipv6/ipv6_sockglue.c:111:13: originally declared here
| net/ipv6/ipv6_sockglue.c:175:16: warning: symbol 'net' shadows an earlier one
| net/ipv6/ipv6_sockglue.c:111:13: originally declared here
| net/ipv6/ip6mr.c:1241:10: warning: symbol 'ret' shadows an earlier one
| net/ipv6/ip6mr.c:1163:6: originally declared here

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