mm/swap: avoid a xa load for swapout path

A variable is never used for swapout path (shadowp is NULL) and compiler
is unable to optimize out the unneeded load since it's a function call.

The was introduced by 3852f676 ("mm/swapcache: support to handle the
shadow entries").

Link: https://lkml.kernel.org/r/20231017011728.37508-1-ryncsn@gmail.comSigned-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swap_state.c

@@ -109,9 +109,9 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
 			goto unlock;
 		for (i = 0; i < nr; i++) {
 			VM_BUG_ON_FOLIO(xas.xa_index != idx + i, folio);
-			old = xas_load(&xas);
-			if (xa_is_value(old)) {
-				if (shadowp)
+			if (shadowp) {
+				old = xas_load(&xas);
+				if (xa_is_value(old))
 					*shadowp = old;
 			}
 			xas_store(&xas, folio);