RDMA/restrack: Simplify restrack tracking in kernel flows

Have a single rdma_restrack_add() that adds an entry, there is no reason
to split the user/kernel here, the rdma_restrack_set_task() is responsible
for this difference.

This patch prepares the code to the future requirement of making restrack
is mandatory for managing ib objects.

Link: https://lore.kernel.org/r/20200922091106.2152715-5-leon@kernel.org


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