sctp: use bitmap_weight

Use bitmap_weight to count the total number of bits set in bitmap.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Sridhar Samudrala <sri@us.ibm.com>
Cc: linux-sctp@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>