BACKPORT: binderfs: add support for feature files

Provide userspace with a mechanism to discover features supported by
the binder driver to refrain from using any unsupported ones in the
first place. Starting with "oneway_spam_detection" only new features
are to be listed under binderfs and all previous ones are assumed to
be supported.

Assuming an instance of binderfs has been mounted at /dev/binderfs,
binder feature files can be found under /dev/binderfs/features/.
Usage example:

  $ mkdir /dev/binderfs
  $ mount -t binder binder /dev/binderfs
  $ cat /dev/binderfs/features/oneway_spam_detection
  1

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20210715031805.1725878-1-cmllamas@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit fc470abf54b2bd6e539065e07905e767b443d719)
Bug: 191910201
Signed-off-by: Carlos Llamas <cmllamas@google.com>
[cmllamas: fix merge conflicts due to missing 095cf502b31e]
Change-Id: Ia5c03aa1881981bee26459e741134b83d5b59693