selinux: allow per-file labelling for binderfs

This patch allows genfscon per-file labeling for binderfs.
This is required to have separate permissions to allow
access to binder, hwbinder and vndbinder devices which are
relocating to binderfs.

Acked-by: Jeff Vander Stoep <jeffv@google.com>
Acked-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>