sunrpc: add a Kconfig option to redirect dfprintk() output to trace buffer

We have a lot of old dprintk() call sites that aren't going anywhere
anytime soon. At the same time, turning them up is a serious burden on
the host due to the console locking overhead.

Add a new Kconfig option that redirects dfprintk() output to the trace
buffer. This is more efficient than logging to the console and allows
for proper interleaving of dprintk and static tracepoint events.

Since using trace_printk() causes scary warnings to pop at boot time,
this new option defaults to "n".

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>