net: qrtr: ns: Change servers radix tree to xarray

There is a use after free scenario while iterating through the servers
radix tree despite the ns being a single threaded process. This can
happen when the radix tree APIs are not synchronized with the
rcu_read_lock() APIs.

Convert the radix tree for servers to xarray to take advantage of the
built in rcu lock usage provided by xarray.

Change-Id: I1d9b017da4efba9d8fc72e4666253060cc7b87e3
Signed-off-by: Chris Lew <clew@codeaurora.org>