dl: Only allow unloading modules that are not dependencies

dl: Only allow unloading modules that are not dependencies

When a module is attempted to be removed its reference counter is always
decremented. This means that repeated rmmod invocations will cause the
module to be unloaded even if another module depends on it.

This may lead to a use-after-free scenario allowing an attacker to execute
arbitrary code and by-pass the UEFI Secure Boot protection.

While being there, add the extern keyword to some function declarations in
that header file.

Fixes: CVE-2020-25632
Reported-by: Chris Coulson <chris.coulson@canonical.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>