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>