FROMLIST: binder: fix death race conditions

(from https://patchwork.kernel.org/patch/9817765/

)

A race existed where one thread could register
a death notification for a node, while another
thread was cleaning up that node and sending
out death notifications for its references,
causing simultaneous access to ref->death
because different locks were held.

Test: boots, manual testing
Change-Id: Iff73312f34f70374f417beba4c4c82dd33cac119
Signed-off-by: Martijn Coenen <maco@google.com>