RDMA/core: Add iWARP protocol statistics attributes in sysfs

This patch adds a sysfs attribute group called "proto_stats" under
/sys/class/infiniband/$device/ and populates this group with protocol
statistics if they exist for a given device.  Currently, only iWARP
stats are defined, but the code is designed to allow InfiniBand
protocol stats if they become available.  These stats are per-device
and more importantly -not- per port.

Details:

- Add union rdma_protocol_stats in ib_verbs.h.  This union allows
  defining transport-specific stats.  Currently only iwarp stats are
  defined.

- Add struct iw_protocol_stats to define the current set of iwarp
  protocol stats.

- Add new ib_device method called get_proto_stats() to return protocol
  statistics.

- Add logic in core/sysfs.c to create iwarp protocol stats attributes
  if the device is an RNIC and has a get_proto_stats() method.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>