iwlwifi: pcie: optimise struct iwl_rx_mem_buffer layout

On 64-bit machines, struct iwl_rx_mem_buffer has a lot of
padding due to the use of pointers after the small items.
Move the list entry before them, and while at it also add
documentation for it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210802170640.6a62255b3df0.I47bb36530a3c2cdbd73454c796ce608ee2a32a6c@changeidSigned-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/pcie/internal.h

@@ -42,6 +42,7 @@ struct iwl_host_cmd;
  * struct iwl_rx_mem_buffer
  * @page_dma: bus address of rxb page
  * @page: driver's pointer to the rxb page
+ * @list: list entry for the membuffer
  * @invalid: rxb is in driver ownership - not owned by HW
  * @vid: index of this rxb in the global table
  * @offset: indicates which offset of the page (in bytes)
@@ -50,10 +51,10 @@ struct iwl_host_cmd;
 struct iwl_rx_mem_buffer {
 	dma_addr_t page_dma;
 	struct page *page;
-	u16 vid;
-	bool invalid;
 	struct list_head list;
 	u32 offset;
+	u16 vid;
+	bool invalid;
 };
 
 /**