IB/core: Move rdma_addr_find_l2_eth_by_grh to core_priv.h

Before commit [1], rdma_addr_find_l2_eth_by_grh() was an exported function
and therefore declaration in include/rdma/ib_addr.h was fine.

But now that its scope is limited to ib_core module, its better to have it
in core_priv.h.

[1] commit 1060f865 ("IB/{core/cm}: Fix generating a return AH for
RoCEE")

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