module: fix the module name length in param_sysfs_builtin

the original code use KOBJ_NAME_LEN for built-in module name length,
that's defined to 20 in linux/kobject.h, but this is not enough appearntly,
many module names are longer than this;
 #define KOBJ_NAME_LEN                   20

another macro is MODULE_NAME_LEN defined in linux/module.h, I think this is
enough for module names:
 #define MODULE_NAME_LEN (64 - sizeof(unsigned long))

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>