IB/core: When marshaling uverbs path, clear unused fields

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

Fixes: dd5f03be ("IB/core: Ethernet L2 attributes in verbs/cm structures")
Reported-by: Aleksey Senin <alekseys@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>