mm/vmalloc: include header for prototype of set_iounmap_nonlazy

make W=1 generates the following warning for mm/vmalloc.c

  mm/vmalloc.c:1599:6: warning: no previous prototype for `set_iounmap_nonlazy' [-Wmissing-prototypes]
   void set_iounmap_nonlazy(void)
        ^~~~~~~~~~~~~~~~~~~

This is an arch-generic function only used by x86.  On other arches, it's
dead code.  Include the header with the definition and make it x86-64
specific.

Link: https://lkml.kernel.org/r/20210520084809.8576-3-mgorman@techsingularity.netSigned-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>

mm/vmalloc.c

@@ -25,6 +25,7 @@
 #include <linux/notifier.h>
 #include <linux/rbtree.h>
 #include <linux/xarray.h>
+#include <linux/io.h>
 #include <linux/rcupdate.h>
 #include <linux/pfn.h>
 #include <linux/kmemleak.h>
@@ -1607,6 +1608,7 @@ static DEFINE_MUTEX(vmap_purge_lock);
 /* for per-CPU blocks */
 static void purge_fragmented_blocks_allcpus(void);
 
+#ifdef CONFIG_X86_64
 /*
  * called before a call to iounmap() if the caller wants vm_area_struct's
  * immediately freed.
@@ -1615,6 +1617,7 @@ void set_iounmap_nonlazy(void)
 {
 	atomic_long_set(&vmap_lazy_nr, lazy_max_pages()+1);
 }
+#endif /* CONFIG_X86_64 */
 
 /*
  * Purges all lazily-freed vmap areas.