net/smc: implement ID-related operations of loopback-ism

This implements operations related to IDs for the loopback-ism device.
loopback-ism uses an Extended GID that is a 128-bit GID instead of the
existing ISM 64-bit GID, and uses the CHID defined with the reserved
value 0xFFFF.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Reviewed-and-tested-by: Jan Karcher <jaka@linux.ibm.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>