IB/hfi1: Fix credit return threshold adjustment

The credit return threshold adjustment on mtu change algorithm does not
take into account all the kernel send contexts that are assigned per VL.
Use the pio send context map to adjust the credit return thresholds for
all the allocated and assigned kernel send contexts based on the MTU
adjustment per VL.

The pio send context map can be changed dynamically based on the actual
number of operational vls which is set by the fabric manager. When this
happens update the credit return threshold values for all the remapped
kernel send contexts.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Jianxin Xiong <jianxin.xiong@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>