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.

Change-Id: I2392eb8075ac0aee51f1749ac398a663853ef4e6
Signed-off-by: Martijn Coenen <maco@google.com>