IB/addr: Constify dst_entry pointer

Make dst_entry pointer as const struct dst_entry* to improve code
readablity to make sure that dst structure fields are not modified by
various functions which are using it.

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