RDMA/core: Check for verbs callbacks before using them

Make sure the providers implement the verbs callbacks before calling
them, otherwise return -EOPNOTSUPP.

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>