GEOM: Skip copyin() for GCTL_PARAM_WR parameters.

Kernel does not read those parameters, so copyin is pointless.

While there, replace some KASSERT()s with CTASSERT()s.

MFC after: 1 month

(cherry picked from commit 01b9c48b5dec8d4a1161e0592f04b3301c81a1f8)