Pull up following revision(s) (requested by hannken in ticket #669):

sys/dev/ccd.c: revision 1.190

Using a ccd(4) with GPT (dk* at ccd*) the disk framework will call
ccdstrategy() -> ccdstart() -> ccdbuffer()  from softint context.

Allocating the buffer with PR_WAITOK here is forbidden.

Change ccdstart() / ccdbuffer() to report failure back to caller and
pass PR_WAITOK / PR_NOWAIT as an additional argument.

Call ccdstart() with PR_NOPWAIT from ccdstrategy() and on error defer
to the kthread.  Call ccdstart() with PR_WAITOK from kthread so requests
from kthread always succeed to allocate the buffers.

Remove the (non working) throttling on low memory as it is no longer needed.

Fixes PR kern/58043 "kernel crash in assert_sleepable() in -current,
dk(4) driver?"

(martin)