IB/rdmavt: Add QP iterator API for QPs

There are currently 3 spots in the qib and hfi1 driver that have
knowledge of the internal QP hash list that should only be in
scope to rdmavt QP code.

Add an iterator API for processing all QPs to hide the
nature of the RCU hashlist.

The API consists of:
- rvt_qp_iter_init()
  * For iterating QPs one at a time for seq_file semantics
- rvt_qp_iter_next()
  * For iterating QPs one at a time for seq_file semantics
- rvt_qp_iter()
  * For iterating all QPs

The first two are used for things like seq_file prints.

The last is for code that just needs to iterate all QPs
in the system.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>