security: add cred argument to security_capable()

commit 6037b715 upstream.

Expand security_capable() to include cred, so that it can be usable in a
wider range of call sites.

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>
[wt: needed by next patch only]
Signed-off-by: Willy Tarreau <w@1wt.eu>