sctp: add SCTP_ADDR_POTENTIALLY_FAILED notification

sctp: add SCTP_ADDR_POTENTIALLY_FAILED notification

SCTP Quick failover draft section 5.1, point 5 has been removed
from rfc7829. Instead, "the sender SHOULD (i) notify the Upper
Layer Protocol (ULP) about this state transition", as said in
section 3.2, point 8.

So this patch is to add SCTP_ADDR_POTENTIALLY_FAILED, defined
in section 7.1, "which is reported if the affected address
becomes PF". Also remove transport cwnd's update when moving
from PF back to ACTIVE , which is no longer in rfc7829 either.

Note that ulp_notify will be set to false if asoc->expose is
not 'enabled', according to last patch.

v2->v3:
  - define SCTP_ADDR_PF SCTP_ADDR_POTENTIALLY_FAILED.
v3->v4:
  - initialize spc_state with SCTP_ADDR_AVAILABLE, as Marcelo suggested.
  - check asoc->pf_expose in sctp_assoc_control_transport(), as Marcelo
    suggested.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>