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

For some strings (e.g. version string), they are permitted to be larger
than PAGE_SIZE (although meaningless), so recommend to use scnprintf()
instead of sprintf().

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