openib_common: set providers mtu to active_mtu instead of 2048

openib_common: set providers mtu to active_mtu instead of 2048

Better out of the box performance when setting mtu to active_mtu
instead of default settings of 2K. The new mtu settings are applied
on a per QP basis and negotiated via CM mtu 8-bit field. One of the
reserved 8 bit CM message fields is used to insure compatibility
with older versions.

If older endpoints are mixed with newer versions it will fallback to
the pre-existing 2K MTU settings, unless overriden by DAPL_IB_MTU.

The change has been made across all providers including ucm, scm, mcm,
and cma (rdma_cm). The mcm provider on a MIC will notify the CCL Proxy
service of a DAPL_IB_MTU override via a new MIX_OP_FLAGS bit
MIX_OP_MTU during the open call.

Signed-off-by: Arlin Davis <arlin.r.davis@intel.com>