This adds a tracepoint that can both observe the nbd request being sent
to the server, as well as modify that request , e.g., setting a flag in
the request that will cause the server to collect detailed tracing data.

The struct request * being handled is included to permit correlation
with the block tracepoints.
Signed-off-by: NMatt Mullins <mmullins@fb.com>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>