fm10k: fix race accessing page->_count

This is illegal to use atomic_set(&page->_count, 2) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/fm10k/fm10k_main.c

@@ -219,11 +219,10 @@ static bool fm10k_can_reuse_rx_page(struct fm10k_rx_buffer *rx_buffer,
 	/* flip page offset to other buffer */
 	rx_buffer->page_offset ^= FM10K_RX_BUFSZ;
 
-	/* since we are the only owner of the page and we need to
-	 * increment it, just set the value to 2 in order to avoid
-	 * an unnecessary locked operation
+	/* Even if we own the page, we are not allowed to use atomic_set()
+	 * This would break get_page_unless_zero() users.
 	 */
-	atomic_set(&page->_count, 2);
+	atomic_inc(&page->_count);
 #else
 	/* move offset up to the next cache line */
 	rx_buffer->page_offset += truesize;