apparmor: Fix failed mount permission check error message

When the mount check fails due to a permission check failure instead
of explicitly at one of the subcomponent checks, AppArmor is reporting
a failure in the flags match. However this is not true and AppArmor
can not attribute the error at this point to any particular component,
and should only indicate the mount failed due to missing permissions.

Fixes: 2ea3ffb7 ("apparmor: add mount mediation")
Signed-off-by: John Johansen <john.johansen@canonical.com>