Use bitmap_weight_and() instead of hand writing it.

This saves a few LoC and is slightly faster, should it mater.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/a29c2348a062408bec45cee2601b2417310e5ea7.1684865809.git.christophe.jaillet@wanadoo.frSigned-off-by: Paolo Abeni <pabeni@redhat.com>