IB/core: Clear AH attr variable to prevent garbage data

During create-ah from userspace, uverbs is sending garbage data in
attr.dmac and attr.vlan_id.  This patch sets attr.dmac and
attr.vlan_id to zero.

Fixes: dd5f03be

 ("IB/core: Ethernet L2 attributes in verbs/cm structures")
Signed-off-by: Devesh Sharma <devesh.sharma@emulex.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>