rds: support for zcopy completion notification

RDS removes a datagram (rds_message) from the retransmit queue when
an ACK is received. The ACK indicates that the receiver has queued
the RDS datagram, so that the sender can safely forget the datagram.
When all references to the rds_message are quiesced, rds_message_purge
is called to release resources used by the rds_message

If the datagram to be removed had pinned pages set up, add
an entry to the rs->rs_znotify_queue so that the notifcation
will be sent up via rds_rm_zerocopy_callback() when the
rds_message is eventually freed by rds_message_purge.

rds_rm_zerocopy_callback() attempts to batch the number of cookies
sent with each notification  to a max of SO_EE_ORIGIN_MAX_ZCOOKIES.
This is achieved by checking the tail skb in the sk_error_queue:
if this has room for one more cookie, the cookie from the
current notification is added; else a new skb is added to the
sk_error_queue. Every invocation of rds_rm_zerocopy_callback() will
trigger a ->sk_error_report to notify the application.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>