kernel/module.c: use scnprintf() instead of sprintf()

For some strings, they are permitted to be larger than PAGE_SIZE, so
need use scnprintf() instead of sprintf(), or it will cause issue.

One case is:

  if a module version is crazy defined (length more than PAGE_SIZE),
  'modinfo' command is still OK (print full contents),
  but for "cat /sys/modules/'modname'/version", will cause issue in kernel.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>