IB/ipoib: Use ib_init_ah_from_path to initialize ah_attr

To support destinations that are not on the local IB subnet, IPoIB
should include the GRH information when constructing an address
handle.  Using the existing ib_init_ah_from_path() call will do this
for us.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>