Add link_layer field port attribute

Add link_layer field port attribute

The new field has three possible values: IBV_LINK_LAYER_UNSPECIFIED,
IBV_LINK_LAYER_INFINIBAND, IBV_LINK_LAYER_ETHERNET.  It can be used by
applications to know the link layer used by the port, which can be
either InfiniBand or Ethernet.

The addition of the new field does not change the size of struct
ibv_port_attr due to alignment of the preceding fields.  Binary
compatibility between the library to applications is kept, since old
apps running over new library do not read this field, and new apps
running over old library will determine the link layer as unspecified
and hence take their IB code path.

The solution was suggested by Roland Dreier <roland@purestorage.com>
and Jason Gunthorpe <jgunthorpe@obsidianresearch.com>

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.co.il>
Signed-off-by: Eli Cohen <eli@mellanox.co.il>