UPSTREAM: binderfs: reserve devices for initial mount

The binderfs instance in the initial ipc namespace will always have a
reserve of 4 binder devices unless explicitly capped by specifying a lower
value via the "max" mount option.
This ensures when binder devices are removed (on accident or on purpose)
they can always be recreated without risking that all minor numbers have
already been used up.

Cc: Todd Kjos <tkjos@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 136497735
(cherry picked from commit 36bdf3cae09df891b191f3955c8e54a2e05d67d0)
Change-Id: I001f305659c37e3b631696712332ae2e21464be8
Signed-off-by: Hridya Valsaraju <hridya@google.com>