Linux 2.6.39 compat: Test if kstrtoul() exists

kstrtoul() exists only after torvalds/linux@33ee3b2e2eb9 in 2.6.39.
Use strict_strtoul() if kstrtoul() doesn't exist.
Note that strict_strtoul() has existed as an alias for kstrtoul()
for a while, but removed in torvalds/linux@3db2e9cdc085.

It looks like RHEL6 (2.6.32 based) has backported kstrtoul(),
and this caused build CI to pass compilation test.
It should fail on vanilla < 2.6.39 kernels or distro kernels without
backport as reported in openzfs#8760.

--
 # grep "kstrtoul(" /lib/modules/2.6.32-754.12.1.el6.x86_64/build/ \
 include/linux/kernel.h >/dev/null
 # echo $?
 0

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes openzfs#8760
Closes openzfs#8761

config/kernel-kstrtoul.m4

@@ -0,0 +1,21 @@
+dnl #
+dnl # 2.6.39 API change
+dnl #
+dnl # 33ee3b2e2eb9 kstrto*: converting strings to integers done (hopefully) right
+dnl #
+dnl # If kstrtoul() doesn't exist, fallback to use strict_strtoul() which has
+dnl # existed since 2.6.25.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_KSTRTOUL], [
+	AC_MSG_CHECKING([whether kstrtoul() exists])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/kernel.h>
+	],[
+		int ret __attribute__ ((unused)) = kstrtoul(NULL, 10, NULL);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_KSTRTOUL, 1, [kstrtoul() exists])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -168,6 +168,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_TOTALHIGH_PAGES
 	ZFS_AC_KERNEL_BLK_QUEUE_DISCARD
 	ZFS_AC_KERNEL_BLK_QUEUE_SECURE_ERASE
+	ZFS_AC_KERNEL_KSTRTOUL
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNEL_MAKE="$KERNEL_MAKE O=$LINUX_OBJ"