IB/mlx4: Convert rej_tmout radix-tree to XArray

Was missed during the initial review of the below patch

Fixes: 227a0e14 ("IB/mlx4: Add support for REJ due to timeout")
Link: https://lore.kernel.org/r/1602253482-6718-1-git-send-email-haakon.bugge@oracle.com


Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>