IB/srpt: Eliminate srpt_find_channel()

In the CM REQ message handler, store the channel pointer in
cm_id->context such that the function srpt_find_channel() is no
longer needed. Additionally, make the CM event messages more
informative.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>