IB/core: Do not resolve VLAN if already resolved

For RoCE, resolution of layer 2 address attributes forces no VLAN if
link-local GIDs are used.  This patch allows applications to choose
the VLAN ID for link-local based RoCE GIDs by setting IB_QP_VID in
their QP attribute mask, and prevents the core from overriding this
choice.

Cc: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>