rxrpc: Add stats procfile and DATA packet stats

Add a procfile, /proc/net/rxrpc/stats, to display some statistics about
what rxrpc has been doing.  Writing a blank line to the stats file will
clear the increment-only counters.  Allocated resource counters don't get
cleared.

Add some counters to count various things about DATA packets, including the
number created, transmitted and retransmitted and the number received, the
number of ACK-requests markings and the number of jumbo packets received.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org