IB/srp: Prevent mapping failures

If both max_sectors and the queue_depth are high enough it can
happen that the MR pool is depleted temporarily. This causes
the SRP initiator to report mapping failures. Although the SRP
initiator recovers from such mapping failures, prevent that
this can happen by allocating more memory regions.

Additionally, only enable memory registration if at least two
pages can be registered per memory region.

Reported-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>
diff --git a/drivers/infiniband/ulp/srp/ib_srp.h b/drivers/infiniband/ulp/srp/ib_srp.h
index a00914c..26bb9b0 100644
--- a/drivers/infiniband/ulp/srp/ib_srp.h
+++ b/drivers/infiniband/ulp/srp/ib_srp.h
@@ -203,6 +203,7 @@
 	unsigned int		scsi_id;
 	unsigned int		sg_tablesize;
 	int			mr_pool_size;
+	int			mr_per_cmd;
 	int			queue_size;
 	int			req_ring_size;
 	int			comp_vector;