config: fix page_mapping test

It always failed from "unused variable" warnings-errors. The resulting
`#define page_mapping(...)` happend to work because it always overrode
the kernel's function prototype, but that's brittle.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#16479

config/kernel-mm-pagemap.m4

@@ -21,8 +21,8 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_MM_PAGE_MAPPING], [
 	ZFS_LINUX_TEST_SRC([page_mapping], [
 		#include <linux/pagemap.h>
 	],[
-		struct page *p = NULL;
-		struct address_space *m = page_mapping(NULL);
+		struct address_space *m;
+		m = page_mapping(NULL);
 	])
 ])
 AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_MAPPING], [