staging: lustre: Replace strtoul with simple_strtoul

Defining and using strtoul in lustre code results in the following
compile warnings (arm64:allmodconfig).

include/linux/libcfs/libcfs_string.h:105:0: warning: "strtoul" redefined
	#define strtoul(str, endp, base) simple_strtoul(str, endp, base)
include/acpi/platform/aclinux.h:122:0: note:
		this is the location of the previous definition
	#define strtoul	simple_strtoul

Remove the definition and use simple_strtoul() directly. Note that we
can not replace simple_strtoul with kstrtoul since the end pointer
is used by the code.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>