net: esp: cleanup esp_output_tail_tcp() in case of unsupported ESPINTCP

xmit() functions should consume skb or return error codes in error
paths.
When the configuration "CONFIG_INET_ESPINTCP" is not set, the
implementation of the function "esp_output_tail_tcp" violates this rule.
The function frees the skb and returns the error code.
This change removes the kfree_skb from both functions, for both
esp4 and esp6.
WARN_ON is added because esp_output_tail_tcp() should never be called if
CONFIG_INET_ESPINTCP is not set.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Fixes: e27cca96 ("xfrm: add espintcp (RFC 8229)")
Signed-off-by: Hagar Hemdan <hagarhem@amazon.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/esp4.c

@@ -239,8 +239,7 @@ static int esp_output_tail_tcp(struct xfrm_state *x, struct sk_buff *skb)
 #else
 static int esp_output_tail_tcp(struct xfrm_state *x, struct sk_buff *skb)
 {
-	kfree_skb(skb);
-
+	WARN_ON(1);
 	return -EOPNOTSUPP;
 }
 #endif

net/ipv6/esp6.c

@@ -256,8 +256,7 @@ static int esp_output_tail_tcp(struct xfrm_state *x, struct sk_buff *skb)
 #else
 static int esp_output_tail_tcp(struct xfrm_state *x, struct sk_buff *skb)
 {
-	kfree_skb(skb);
-
+	WARN_ON(1);
 	return -EOPNOTSUPP;
 }
 #endif