ixgbe: fix end of loop test in ixgbe_set_vf_macvlan()

The list iterator in a list_for_each_entry() loop can never be NULL.
If the loop exits without hitting a break then the iterator points
to an offset off the list head and dereferencing it is an out of
bounds access.

Before we transitioned to using list_for_each_entry() loops, then
it was possible for "entry" to be NULL and the comments mention
this.  I have updated the comments to match the new code.

Fixes: c1fec890 ("ethernet/intel: Use list_for_each_entry() helper")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -640,6 +640,7 @@ static int ixgbe_set_vf_macvlan(struct ixgbe_adapter *adapter,
 				int vf, int index, unsigned char *mac_addr)
 {
 	struct vf_macvlans *entry;
+	bool found = false;
 	int retval = 0;
 
 	if (index <= 1) {
@@ -661,22 +662,22 @@ static int ixgbe_set_vf_macvlan(struct ixgbe_adapter *adapter,
 	if (!index)
 		return 0;
 
-	entry = NULL;
-
 	list_for_each_entry(entry, &adapter->vf_mvs.l, l) {
-		if (entry->free)
+		if (entry->free) {
+			found = true;
 			break;
+		}
 	}
 
 	/*
 	 * If we traversed the entire list and didn't find a free entry
-	 * then we're out of space on the RAR table.  Also entry may
-	 * be NULL because the original memory allocation for the list
-	 * failed, which is not fatal but does mean we can't support
-	 * VF requests for MACVLAN because we couldn't allocate
-	 * memory for the list management required.
+	 * then we're out of space on the RAR table.  It's also possible
+	 * for the &adapter->vf_mvs.l list to be empty because the original
+	 * memory allocation for the list failed, which is not fatal but does
+	 * mean we can't support VF requests for MACVLAN because we couldn't
+	 * allocate memory for the list management required.
 	 */
-	if (!entry || !entry->free)
+	if (!found)
 		return -ENOSPC;
 
 	retval = ixgbe_add_mac_filter(adapter, mac_addr, vf);