s390/lcs: Fix return type of lcs_start_xmit()

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed. A
proposed warning in clang aims to catch these at compile time, which
reveals:

  drivers/s390/net/lcs.c:2090:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .ndo_start_xmit         = lcs_start_xmit,
                                    ^~~~~~~~~~~~~~
  drivers/s390/net/lcs.c:2097:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .ndo_start_xmit         = lcs_start_xmit,
                                    ^~~~~~~~~~~~~~

->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
'netdev_tx_t', not 'int'. Adjust the return type of lcs_start_xmit() to
match the prototype's to resolve the warning and potential CFI failure,
should s390 select ARCH_SUPPORTS_CFI_CLANG in the future.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/lcs.c

@@ -1519,9 +1519,8 @@ lcs_txbuffer_cb(struct lcs_channel *channel, struct lcs_buffer *buffer)
 /*
  * Packet transmit function called by network stack
  */
-static int
-__lcs_start_xmit(struct lcs_card *card, struct sk_buff *skb,
-		 struct net_device *dev)
+static netdev_tx_t __lcs_start_xmit(struct lcs_card *card, struct sk_buff *skb,
+				    struct net_device *dev)
 {
 	struct lcs_header *header;
 	int rc = NETDEV_TX_OK;
@@ -1582,8 +1581,7 @@ __lcs_start_xmit(struct lcs_card *card, struct sk_buff *skb,
 	return rc;
 }
 
-static int
-lcs_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t lcs_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct lcs_card *card;
 	int rc;