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 #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 <[email protected]>
Reviewed-by: loli10K <[email protected]>
Signed-off-by: Tomohiro Kusumi <[email protected]>
Closes #8760 
Closes #8761 

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