scsi: RDMA/srpt: Rework I/O context allocation

Instead of maintaining a list of free I/O contexts, use an sbitmap data
structure to track which I/O contexts are in use and which are free. This
makes the ib_srpt driver more consistent with other LIO drivers.

Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>