config/kmap_atomic: initialise test data

6.18 changes kmap_atomic() to take a const pointer. This is no problem
for the places we use it, but Clang fails the test due to a warning
about being unable to guarantee that uninitialised data will definitely
not change. Easily solved by forcibly initialising it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#17954

Author: robn

config/kernel-kmap-atomic-args.m4

@@ -7,7 +7,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_KMAP_ATOMIC_ARGS], [
 	ZFS_LINUX_TEST_SRC([kmap_atomic], [
 		#include <linux/pagemap.h>
 	],[
-		struct page page;
+		struct page page = {};
 		kmap_atomic(&page);
 	])
 ])