utsname: simplify one-level sysctl registration for uts_kern_table

There is no need to declare an extra tables to just create directory,
this can be easily be done with a prefix path with register_sysctl().

Simplify this registration.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>