kernel -- ccd: Concatenated disk MPLOCK rework.

* Remove freelist of ccdbuf structures; kmalloc is good enough for them,
  removes a global list.

* Rework ccdlock/ccdunlock to use a real lockmgr lock rather than a
  hand-rolled "lock" in the flags field.

* Mark ccd disk D_MPSAFE; calls disk strategy routines without the MPLOCK.

* Remove mplock from ccdiodone biodone callback; use ccdlock() on the
  softcontroller instead.

* Remove unneeded critical section around ccdstart and in ccdioctl.

* Remove mplock2.h.

* Remove cb_freenext linkage from ccd buffers.