net: qrtr: free memory on error path in radix_tree_insert()

Function radix_tree_insert() returns errors if the node hasn't
been initialized and added to the tree.

"kfree(node)" and return value "NULL" of node_get() help
to avoid using unclear node in other calls.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Cc: <stable@vger.kernel.org> # 5.7
Fixes: 0c2204a4

 ("net: qrtr: Migrate nameservice to kernel from userspace")
Signed-off-by: Natalia Petrova <n.petrova@fintech.ru>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/20230125134831.8090-1-n.petrova@fintech.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>