IB/core: When marshaling ucma path from user-space, clear unused fields

When marshaling a user path to the kernel struct ib_sa_path, we need
to zero smac and dmac and set the vlan id to the "no vlan" value.

This is to ensure that Ethernet attributes are not used with
InfiniBand QPs.

Fixes: dd5f03be ("IB/core: Ethernet L2 attributes in verbs/cm structures")
Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>