RDMA/srp: Honor the max_send_sge device attribute

Instead of assuming that max_send_sge >= 3, restrict the number of scatter
gather elements to what is supported by the RDMA adapter.

Link: https://lore.kernel.org/r/20190930231707.48259-8-bvanassche@acm.org


Cc: Honggang LI <honli@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>