net: qrtr: ns: Fix the incorrect usage of rcu_read_lock()

The rcu_read_lock() is not supposed to lock the kernel_sendmsg() API
since it has the lock_sock() in qrtr_sendmsg() which will sleep. Hence,
fix it by excluding the locking for kernel_sendmsg().

While at it, let's also use radix_tree_deref_retry() to confirm the
validity of the pointer returned by radix_tree_deref_slot() and use
radix_tree_iter_resume() to resume iterating the tree properly before
releasing the lock as suggested by Doug.

Fixes: a7809ff9 ("net: qrtr: ns: Protect radix_tree_deref_slot() using rcu read locks")
Reported-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Alex Elder <elder@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>