FROMLIST: binder: protect against stale pointers in print_binder_transaction

(from https://patchwork.kernel.org/patch/9817761/

)

When printing transactions there were several race conditions
that could cause a stale pointer to be deferenced. Fixed by
reading the pointer once and using it if valid (which is
safe). The transaction buffer also needed protection via proc
lock, so it is only printed if we are holding the correct lock.

Change-Id: I9a03129e08eaab4b8a5646eecafaf10e343dbdea
Signed-off-by: Todd Kjos <tkjos@google.com>