IB/{core, ipoib}: Simplify ib_find_gid to search only for IB link layer

Currently there are no users of ib_find_gid for RoCE transport. It is
only used by IPoIB.
Therefore its simplified to ignore RoCE ports and GID type check which
was previously done for every port.

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