IB/qib: Improve SDMA performance

1. The code accepts chunks of messages, and splits the chunk into
   packets when converting packets into sdma queue entries.  Adjacent
   packets will use user buffer pages smartly to avoid pinning the
   same page multiple times.

2. Instead of discarding all the work when SDMA queue is full, the
   work is saved in a pending queue.  Whenever there are enough SDMA
   queue free entries, pending queue is directly put onto SDMA queue.

3. An interrupt handler is used to progress this pending queue.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>

[ Fixed up sparse warnings.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>