IB/core: Refactor ib_init_ah_attr_from_path() for RoCE

Resolving route for RoCE for a path record is needed only for the
received CM requests.
Therefore,
(a) ib_init_ah_attr_from_path() is refactored first to isolate the
code of resolving route.
(b) Setting dlid, path bits is not needed for RoCE.

Additionally ah attribute initialization is done from the path record
entry, so it is better to refer to path record entry type for
different link layer instead of ah attribute type while initializing
ah attribute itself.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>