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.