mm/vmscan.c: fix unsequenced modification and access warning

Clang and its -Wunsequenced emits a warning

  mm/vmscan.c:2961:25: error: unsequenced modification and access to 'gfp_mask' [-Wunsequenced]
                  .gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
                                        ^

While it is not clear to me whether the initialization code violates the
specification (6.7.8 par 19 (ISO/IEC 9899) looks like it disagrees) the
code is quite confusing and worth cleaning up anyway.  Fix this by
reusing sc.gfp_mask rather than the updated input gfp_mask parameter.

Link: http://lkml.kernel.org/r/20170510154030.10720-1-nick.desaulniers@gmail.com
Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2967,7 +2967,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 	unsigned long nr_reclaimed;
 	struct scan_control sc = {
 		.nr_to_reclaim = SWAP_CLUSTER_MAX,
-		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
+		.gfp_mask = current_gfp_context(gfp_mask),
 		.reclaim_idx = gfp_zone(gfp_mask),
 		.order = order,
 		.nodemask = nodemask,
@@ -2982,12 +2982,12 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 	 * 1 is returned so that the page allocator does not OOM kill at this
 	 * point.
 	 */
-	if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
+	if (throttle_direct_reclaim(sc.gfp_mask, zonelist, nodemask))
 		return 1;
 
 	trace_mm_vmscan_direct_reclaim_begin(order,
 				sc.may_writepage,
-				gfp_mask,
+				sc.gfp_mask,
 				sc.reclaim_idx);
 
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
@@ -3774,17 +3774,16 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	const unsigned long nr_pages = 1 << order;
 	struct task_struct *p = current;
 	struct reclaim_state reclaim_state;
-	int classzone_idx = gfp_zone(gfp_mask);
 	unsigned int noreclaim_flag;
 	struct scan_control sc = {
 		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
-		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
+		.gfp_mask = current_gfp_context(gfp_mask),
 		.order = order,
 		.priority = NODE_RECLAIM_PRIORITY,
 		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
 		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
 		.may_swap = 1,
-		.reclaim_idx = classzone_idx,
+		.reclaim_idx = gfp_zone(gfp_mask),
 	};
 
 	cond_resched();
@@ -3795,7 +3794,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	 */
 	noreclaim_flag = memalloc_noreclaim_save();
 	p->flags |= PF_SWAPWRITE;
-	lockdep_set_current_reclaim_state(gfp_mask);
+	lockdep_set_current_reclaim_state(sc.gfp_mask);
 	reclaim_state.reclaimed_slab = 0;
 	p->reclaim_state = &reclaim_state;