binder: separate out binder_alloc functions

Continuation of splitting the binder allocator from the binder
driver. Separate binder_alloc functions from normal binder
functions. Protect the allocator with a separate mutex.

Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>