RDMA/restrack: Convert internal DB from hash to XArray

The additions of .doit callbacks posses new access pattern to the resource
entries by some user visible index. Back then, the legacy DB was
implemented as hash because per-index access wasn't needed and XArray
wasn't accepted yet.

Acceptance of XArray together with per-index access requires the refresh
of DB implementation.

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