ixgbe/ixgbevf: fix XFRM_ALGO dependency

Based on the original work from Arnd Bergmann.

When XFRM_ALGO is not enabled, the new ixgbe IPsec code produces a
link error:

drivers/net/ethernet/intel/ixgbe/ixgbe_ipsec.o: In function `ixgbe_ipsec_vf_add_sa':
ixgbe_ipsec.c:(.text+0x1266): undefined reference to `xfrm_aead_get_byname'

Simply selecting XFRM_ALGO from here causes circular dependencies, so
to fix it, we probably want this slightly more complex solution that is
similar to what other drivers with XFRM offload do:

A separate Kconfig symbol now controls whether we include the IPsec
offload code. To keep the old behavior, this is left as 'default y'. The
dependency in XFRM_OFFLOAD still causes a circular dependency but is
not actually needed because this symbol is not user visible, so removing
that dependency on top makes it all work.

CC: Arnd Bergmann <arnd@arndb.de>
CC: Shannon Nelson <shannon.nelson@oracle.com>
Fixes: eda0333ac2 ("ixgbe: add VF IPsec management")
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -8694,7 +8694,7 @@ netdev_tx_t ixgbe_xmit_frame_ring(struct sk_buff *skb,
 
 #endif /* IXGBE_FCOE */
 
-#ifdef CONFIG_XFRM_OFFLOAD
+#ifdef CONFIG_IXGBE_IPSEC
 	if (skb->sp && !ixgbe_ipsec_tx(tx_ring, first, &ipsec_tx))
 		goto out_drop;
 #endif
@@ -10190,7 +10190,7 @@ ixgbe_features_check(struct sk_buff *skb, struct net_device *dev,
 	 * the TSO, so it's the exception.
 	 */
 	if (skb->encapsulation && !(features & NETIF_F_TSO_MANGLEID)) {
-#ifdef CONFIG_XFRM_OFFLOAD
+#ifdef CONFIG_IXGBE_IPSEC
 		if (!skb->sp)
 #endif
 			features &= ~NETIF_F_TSO;
@@ -10883,7 +10883,7 @@ skip_sriov:
 	if (hw->mac.type >= ixgbe_mac_82599EB)
 		netdev->features |= NETIF_F_SCTP_CRC;
 
-#ifdef CONFIG_XFRM_OFFLOAD
+#ifdef CONFIG_IXGBE_IPSEC
 #define IXGBE_ESP_FEATURES	(NETIF_F_HW_ESP | \
 				 NETIF_F_HW_ESP_TX_CSUM | \
 				 NETIF_F_GSO_ESP)