IB/srp: Enforce protocol limit on srp_sg_tablesize

The current SRP initiator will allow unlimited s/g entries in the
indirect descriptors lists, but the entry count field in the SRP_CMD
request is 8 bits, so setting srp_sg_tablesize too large will open the
possibility of wrapping the count and generating invalid requests.

Clamp srp_sg_tablesize to the protocol limits to prevent surprises.

Reported by Martin W. Schlining III <mschlining@datadirectnet.com>.
Signed-off-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -68,7 +68,7 @@ static int srp_max_iu_len;
 
 module_param(srp_sg_tablesize, int, 0444);
 MODULE_PARM_DESC(srp_sg_tablesize,
-		 "Max number of gather/scatter entries per I/O (default is 12)");
+		 "Max number of gather/scatter entries per I/O (default is 12, max 255)");
 
 static int topspin_workarounds = 1;
 
@@ -2138,6 +2138,11 @@ static int __init srp_init_module(void)
 {
 	int ret;
 
+	if (srp_sg_tablesize > 255) {
+		printk(KERN_WARNING PFX "Clamping srp_sg_tablesize to 255\n");
+		srp_sg_tablesize = 255;
+	}
+
 	ib_srp_transport_template =
 		srp_attach_transport(&ib_srp_transport_functions);
 	if (!ib_srp_transport_template)