module: tracking: Keep a record of tainted unloaded modules only

This ensures that no module record/or entry is added to the
unloaded_tainted_modules list if it does not carry a taint.

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Fixes: 99bd9956

 ("module: Introduce module unload taint tracking")
Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>