ioat: Allocate memory for ring resize sanely

Add a new flag for DMA operations, DMA_NO_WAIT. It behaves much like
other NOWAIT flags -- if queueing an operation would sleep, abort and
return NULL instead.

When growing the internal descriptor ring, the memory allocation is
performed outside of all locks. A lock-protected flag is used to avoid
duplicated work. Threads that cannot sleep and attempt to queue
operations when the descriptor ring is full allocate a larger ring with
M_NOWAIT, or bail if that fails.

ioat_reserve_space() could become an external API if is important to
callers that they have room for a sequence of operations, or that those
operations succeed each other directly in the hardware ring.

This patch splits the internal head index (->head) from the hardware's
head-of-chain (DMACOUNT) register (->hw_head). In the future, for
simplicity's sake, we could drop the 'ring' array entirely and just use
a linked list (with head and tail pointers rather than indices).

Suggested by: Witness
Sponsored by: EMC / Isilon Storage Division