[pnfs] Add partial backchannel support to the client

[Ricardo Labiaga]

Hi,

I've reworked the initial backchannel patch set with the comments
provided by Benny, Trond, and Tom Talpey.  This patch set is the
resubmission of the first part of the backchannel support for the
client.  It's based on the 2.6.18 backchannel.  This initial part of the
functionality implements the detection of RPC calls over an existing
connection and queues them for processing by an NFSv4.1 backchannel RPC
service.

The processing of the request and the reply to the server will be
implemented as a separate patch set.

Patches 5 and 6 deal with the pre-allocation methodology suggested, and
xs_tcp_read_callback() in patch 9 uses the resources.  Because callbacks
are not expected to cause significant latency, only one callback is
allowed at a time, and only resources for a single callback are
pre-allocated.  If the resource is not available - because there is
another callback in progress, the request is dropped and it's up to the
caller to close the connection to retry if necessary.  Note that the
number of callbacks is negotiated during session establishment.

Thanks,

- ricardo