LSM: LoadPin for kernel file loading restrictions

This LSM enforces that kernel-loaded files (modules, firmware, etc)
must all come from the same filesystem, with the expectation that
such a filesystem is backed by a read-only device such as dm-verity
or CDROM. This allows systems that have a verified and/or unchangeable
filesystem to enforce module and firmware loading restrictions without
needing to sign the files individually.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>