BACKPORT: binder: use euid from cred instead of using task

Save the 'struct cred' associated with a binder process
at initial open to avoid potential race conditions
when converting to an euid.

Set a transaction's sender_euid from the 'struct cred'
saved at binder_open() instead of looking up the euid
from the binder proc's 'struct task'. This ensures
the euid is associated with the security context that
of the task that opened binder.

Cc: stable@vger.kernel.org # 4.4+
Fixes: 457b9a6f ("Staging: android: add binder driver")
Signed-off-by: Todd Kjos <tkjos@google.com>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Suggested-by: Jann Horn <jannh@google.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Bug: 200688826
(cherry picked from commit 29bc22ac5e5bc63275e850f0c8fc549e3d0e306b)
[ refactored to avoid changing KMI: struct binder_proc ]
Change-Id: Icaf996a7f5543b7d6943dff3cbe89bfc3614044c