IB/srp: Remove the memory registration backtracking code

Mapping a discontiguous sg-list requires multiple memory regions
and hence can exhaust the memory region pool. The SRP initiator
already handles this by temporarily reducing the queue depth. This
means that it is safe to remove the memory registration backtracking
code. This patch has been tested with direct I/O sizes up to 256 MB.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>