IB/srp: Avoid that sg_reset -d ${srp_device} triggers an infinite loop

Use different loop variables for the inner and outer loop. This avoids
that an infinite loop occurs if there are more RDMA channels than
target->req_ring_size.

Fixes: d92c0da7 ("IB/srp: Add multichannel support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>