nfsd: track the main opcode for callbacks

Keep track of the "main" opcode for the callback, and display it in the
tracepoint. This makes it simpler to discern what's happening when there
is more than one callback in flight.

The one special case is the CB_NULL RPC. That's not a CB_COMPOUND
opcode, so designate the value 0 for that.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>