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.com


Signed-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>