rtnetlink: Fix null-ptr-deref in rtnl_newlink

In rtnl_newlink(), NULL check is performed on m_ops however member of
ops is accessed. Fixed by accessing member of m_ops instead of ops.

[  345.432629] BUG: KASAN: null-ptr-deref in rtnl_newlink+0x400/0x1110
[  345.432629] Read of size 4 at addr 0000000000000088 by task ip/986
[  345.432629]
[  345.432629] CPU: 1 PID: 986 Comm: ip Not tainted 4.17.0-rc6+ #9
[  345.432629] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[  345.432629] Call Trace:
[  345.432629]  dump_stack+0xc6/0x150
[  345.432629]  ? dump_stack_print_info.cold.0+0x1b/0x1b
[  345.432629]  ? kasan_report+0xb4/0x410
[  345.432629]  kasan_report.cold.4+0x8f/0x91
[  345.432629]  ? rtnl_newlink+0x400/0x1110
[  345.432629]  rtnl_newlink+0x400/0x1110
[...]

Fixes: ccf8dbcd ("rtnetlink: Remove VLA usage")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2936,7 +2936,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 
 		if (m_ops) {
-			if (ops->slave_maxtype > RTNL_SLAVE_MAX_TYPE)
+			if (m_ops->slave_maxtype > RTNL_SLAVE_MAX_TYPE)
 				return -EINVAL;
 
 			if (m_ops->slave_maxtype &&