Linux 2.6.39 compat: Test if kstrtoul() exists

kstrtoul() exists only after torvalds/linux@33ee3b2e2e 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@3db2e9cdc0.

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 #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 #8760 
Closes #8761

include/spl/sys/strings.h

@@ -28,4 +28,8 @@
 #define	bcopy(src, dest, size)		memmove(dest, src, size)
 #define	bcmp(src, dest, size)		memcmp((src), (dest), (size_t)(size))
 
+#ifndef HAVE_KSTRTOUL
+#define	kstrtoul strict_strtoul
+#endif
+
 #endif	/* _SPL_SYS_STRINGS_H */