gro: Fix inconsistent indenting

Eliminate the follow smatch warning:

net/ipv6/ip6_offload.c:249 ipv6_gro_receive() warn: inconsistent
indenting.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>