zsmalloc: micro-optimize zs_object_copy()

A micro-optimization.  Avoid additional branching and reduce (a bit)
registry pressure (f.e.  s_off += size; d_off += size; may be calculated
twise: first for >= PAGE_SIZE check and later for offset update in "else"
clause).

scripts/bloat-o-meter shows some improvement

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-10 (-10)
function                          old     new   delta
zs_object_copy                    550     540     -10
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -1537,7 +1537,12 @@ static void zs_object_copy(unsigned long src, unsigned long dst,
 		if (written == class->size)
 			break;
 
-		if (s_off + size >= PAGE_SIZE) {
+		s_off += size;
+		s_size -= size;
+		d_off += size;
+		d_size -= size;
+
+		if (s_off >= PAGE_SIZE) {
 			kunmap_atomic(d_addr);
 			kunmap_atomic(s_addr);
 			s_page = get_next_page(s_page);
@@ -1546,21 +1551,15 @@ static void zs_object_copy(unsigned long src, unsigned long dst,
 			d_addr = kmap_atomic(d_page);
 			s_size = class->size - written;
 			s_off = 0;
-		} else {
-			s_off += size;
-			s_size -= size;
 		}
 
-		if (d_off + size >= PAGE_SIZE) {
+		if (d_off >= PAGE_SIZE) {
 			kunmap_atomic(d_addr);
 			d_page = get_next_page(d_page);
 			BUG_ON(!d_page);
 			d_addr = kmap_atomic(d_page);
 			d_size = class->size - written;
 			d_off = 0;
-		} else {
-			d_off += size;
-			d_size -= size;
 		}
 	}