openvswitch: fix flow actions reallocation

The flow action buffer can be resized if it's not big enough to contain
all the requested flow actions. However, this resize doesn't take into
account the new requested size, the buffer is only increased by a factor
of 2x. This might be not enough to contain the new data, causing a
buffer overflow, for example:

[   42.044472] =============================================================================
[   42.045608] BUG kmalloc-96 (Not tainted): Redzone overwritten
[   42.046415] -----------------------------------------------------------------------------

[   42.047715] Disabling lock debugging due to kernel taint
[   42.047716] INFO: 0x8bf2c4a5-0x720c0928. First byte 0x0 instead of 0xcc
[   42.048677] INFO: Slab 0xbc6d2040 objects=29 used=18 fp=0xdc07dec4 flags=0x2808101
[   42.049743] INFO: Object 0xd53a3464 @offset=2528 fp=0xccdcdebb

[   42.050747] Redzone 76f1b237: cc cc cc cc cc cc cc cc                          ........
[   42.051839] Object d53a3464: ...

net/openvswitch/flow_netlink.c

@@ -2306,14 +2306,14 @@ static struct nlattr *reserve_sfa_size(struct sw_flow_actions **sfa,
 
 	struct sw_flow_actions *acts;
 	int new_acts_size;
-	int req_size = NLA_ALIGN(attr_len);
+	size_t req_size = NLA_ALIGN(attr_len);
 	int next_offset = offsetof(struct sw_flow_actions, actions) +
 					(*sfa)->actions_len;
 
 	if (req_size <= (ksize(*sfa) - next_offset))
 		goto out;
 
-	new_acts_size = ksize(*sfa) * 2;
+	new_acts_size = max(next_offset + req_size, ksize(*sfa) * 2);
 
 	if (new_acts_size > MAX_ACTIONS_BUFSIZE) {
 		if ((MAX_ACTIONS_BUFSIZE - next_offset) < req_size) {