ip_gre: clean up some inconsistent indenting

No functional modification involved.

net/ipv4/ip_gre.c:192 ipgre_err() warn: inconsistent indenting.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5375


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