kernel: Add noaudit variant of ns_capable()

kernel: Add noaudit variant of ns_capable()

When checking the current cred for a capability in a specific user
namespace, it isn't always desirable to have the LSMs audit the check.
This patch adds a noaudit variant of ns_capable() for when those
situations arise.

The common logic between ns_capable() and the new ns_capable_noaudit()
is moved into a single, shared function to keep duplicated code to a
minimum and ease maintainability.

Signed-off-by: Tyler Hicks <[email protected]>
Acked-by: Serge E. Hallyn <[email protected]>
Signed-off-by: James Morris <[email protected]>