sctp: move sk_route_caps check and set into sctp_outq_flush_transports

The sk's sk_route_caps is set in sctp_packet_config, and later it
only needs to change when traversing the transport_list in a loop,
as the dst might be changed in the tx path.

So move sk_route_caps check and set into sctp_outq_flush_transports
from sctp_packet_transmit. This also fixes a dst leak reported by
Chen Yi:

  https://bugzilla.kernel.org/show_bug.cgi?id=212227



As calling sk_setup_caps() in sctp_packet_transmit may also set the
sk_route_caps for the ctrl sock in a netns. When the netns is being
deleted, the ctrl sock's releasing is later than dst dev's deleting,
which will cause this dev's deleting to hang and dmesg error occurs:

  unregister_netdevice: waiting for xxx to become free. Usage count = 1

Reported-by: Chen Yi <yiche@redhat.com>
Fixes: bcd623d8

 ("sctp: call sk_setup_caps in sctp_packet_transmit instead")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>