mm/swap: make swap_address_space an inline function

make W=1 generates the following warning in page_mapping() for allnoconfig

  mm/util.c:700:15: warning: variable `entry' set but not used [-Wunused-but-set-variable]
     swp_entry_t entry;
                 ^~~~~

swap_address is a #define on !CONFIG_SWAP configurations.  Make the helper
an inline function to suppress the warning, add type checking and to apply
any side-effects in the parameter list.

Link: https://lkml.kernel.org/r/20210520084809.8576-12-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -537,7 +537,11 @@ static inline void put_swap_device(struct swap_info_struct *si)
 {
 }
 
-#define swap_address_space(entry)		(NULL)
+static inline struct address_space *swap_address_space(swp_entry_t entry)
+{
+	return NULL;
+}
+
 #define get_nr_swap_pages()			0L
 #define total_swap_pages			0L
 #define total_swapcache_pages()			0UL