apparmor: reserve and mask off the top 8 bits of the base field

The top 8 bits of the base field have never been used, in fact can't
be used, by the current 'dfa16' format.  However they will be used in the
future as flags, so mask them off when using base as an index value.

Note: the use of the top 8 bits, without masking is trapped by the verify
      checks that base entries are within the size bounds.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <kees@ubuntu.com>