IB/ehca: Use WQE offset instead of WQE addr for pending work reqs

This is a patch for ehca to fix a bug in prepare_sqe_to_rts(), which
used WQE address to iterate pending work requests.  This might cause
an access violation since the queue pages can not be assumed to follow
each other consecutively.  Thus, this patch introduces a few queue
functions to determine WQE offset based on its address and uses WQE
offset to iterate the pending work requests.

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>