MFC r297858: Allocate separate DMA area for synchronous IOCB execution.

MFC r297858: Allocate separate DMA area for synchronous IOCB execution.

Usually IOCBs should be put on queue for asynchronous processing and should
not require additional DMA memory.  But there are some cases like aborts and
resets that for external reasons has to be synchronous.  Give those cases
separate 2*64 byte DMA area to decouple them from other DMA scratch area
users, using it for asynchronous requests.

git-svn-id: svn://svn.freebsd.org/base/stable/10@298966 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f