iavf: fix dereference of null rx_buffer pointer

[ Upstream commit 9fe06a51287b2d41baef7ece94df34b5abf19b90 ]

A recent commit efa14c3985828d ("iavf: allow null RX descriptors") added
a null pointer sanity check on rx_buffer, however, rx_buffer is being
dereferenced before that check, which implies a null pointer dereference
bug can potentially occur.  Fix this by only dereferencing rx_buffer
until after the null pointer check.

Addresses-Coverity: ("Dereference before null check")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>

drivers/net/ethernet/intel/i40evf/i40e_txrx.c

@@ -1291,7 +1291,7 @@ static struct sk_buff *i40e_construct_skb(struct i40e_ring *rx_ring,
 					  struct i40e_rx_buffer *rx_buffer,
 					  unsigned int size)
 {
-	void *va = page_address(rx_buffer->page) + rx_buffer->page_offset;
+	void *va;
 #if (PAGE_SIZE < 8192)
 	unsigned int truesize = i40e_rx_pg_size(rx_ring) / 2;
 #else
@@ -1301,6 +1301,7 @@ static struct sk_buff *i40e_construct_skb(struct i40e_ring *rx_ring,
 	struct sk_buff *skb;
 
 	/* prefetch first cache line of first page */
+	va = page_address(rx_buffer->page) + rx_buffer->page_offset;
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
 	prefetch(va + L1_CACHE_BYTES);
@@ -1355,7 +1356,7 @@ static struct sk_buff *i40e_build_skb(struct i40e_ring *rx_ring,
 				      struct i40e_rx_buffer *rx_buffer,
 				      unsigned int size)
 {
-	void *va = page_address(rx_buffer->page) + rx_buffer->page_offset;
+	void *va;
 #if (PAGE_SIZE < 8192)
 	unsigned int truesize = i40e_rx_pg_size(rx_ring) / 2;
 #else
@@ -1365,6 +1366,7 @@ static struct sk_buff *i40e_build_skb(struct i40e_ring *rx_ring,
 	struct sk_buff *skb;
 
 	/* prefetch first cache line of first page */
+	va = page_address(rx_buffer->page) + rx_buffer->page_offset;
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
 	prefetch(va + L1_CACHE_BYTES);