userns: Add kuid_t and kgid_t and associated infrastructure in uidgid.h

Start distinguishing between internal kernel uids and gids and
values that userspace can use.  This is done by introducing two
new types: kuid_t and kgid_t.  These types and their associated
functions are infrastructure are declared in the new header
uidgid.h.

Ultimately there will be a different implementation of the mapping
functions for use with user namespaces.  But to keep it simple
we introduce the mapping functions first to separate the meat
from the mechanical code conversions.

Export overflowuid and overflowgid so we can use from_kuid_munged
and from_kgid_munged in modular code.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: stratosk <stratosk@semaphore.gr>
userns: Add a Kconfig option to enforce strict kuid and kgid type checks

Make it possible to easily switch between strong mandatory
type checks and relaxed type checks so that the code can
easily be tested with the type checks and then built
with the strong type checks disabled so the resulting
code can be used.

Require strong mandatory type checks when enabling the user namespace.
It is very simple to make a typo and use the wrong type allowing
conversions to/from userspace values to be bypassed by accident,
the strong type checks prevent this.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: stratosk <stratosk@semaphore.gr>