binderfs: implement "max" mount option

Since binderfs can be mounted by userns root in non-initial user namespaces
some precautions are in order. First, a way to set a maximum on the number
of binder devices that can be allocated per binderfs instance and second, a
way to reserve a reasonable chunk of binderfs devices for the initial ipc
namespace.
A first approach as seen in [1] used sysctls similiar to devpts but was
shown to be flawed (cf. [2] and [3]) since some aspects were unneeded. This
is an alternative approach which avoids sysctls completely and instead
switches to a single mount option.

Starting with this commit binderfs instances can be mounted with a limit on
the number of binder devices that can be allocated. The max=<count> mount
option serves as a per-instance limit. If max=<count> is set then only
<count> number of binder devices can be allocated in this binderfs
instance.

This allows to safely bind-mount binderfs instances into unprivileged user
namespaces since userns root in a non-initial user namespace cannot change
the mount option as long as it does not own the mount namespace the
binderfs mount was created in and hence cannot drain the host of minor
device numbers

[1]: https://lore.kernel.org/lkml/20181221133909.18794-1-christian@brauner.io/
[2]; https://lore.kernel.org/lkml/20181221163316.GA8517@kroah.com/
[3]: https://lore.kernel.org/lkml/CAHRSSEx+gDVW4fKKK8oZNAir9G5icJLyodO8hykv3O0O1jt2FQ@mail.gmail.com/
[4]: https://lore.kernel.org/lkml/20181221192044.5yvfnuri7gdop4rs@brauner.io/

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>