IB: Update references to libibverbs

These days the userspace comes from rdma-core, revise references
in the kernel to point to the current repository.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>