util/interval-tree: Use qatomic_set_mb in rb_link_node

util/interval-tree: Use qatomic_set_mb in rb_link_node

Ensure that the stores to rb_left and rb_right are complete before
inserting the new node into the tree.  Otherwise a concurrent reader
could see garbage in the new leaf.

Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 4c8baa02d36379507afd17bdea87aabe0aa32ed3)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: s/qatomic_set_mb/qatomic_mb_set/ for 8.0 - it was renamed later)