RDMA/srp: Make the channel count configurable per target

Increase the flexibility of the SRP initiator driver by making the channel
count configurable per target instead of only providing a kernel module
parameter for configuring the channel count.

Link: https://lore.kernel.org/r/20200525172212.14413-2-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>