RDMA/srp: Rework handling of the maximum information unit length

RDMA/srp: Rework handling of the maximum information unit length

Move the maximum initiator to target information unit length parameter
from struct srp_target_port into struct srp_rdma_ch. This patch does
not change any functionality but makes the next patch easier to read.

Cc: Sergey Gorenko <sergeygo@mellanox.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>