nbd: add tracepoints for send/receive timing

This adds four tracepoints to nbd, enabling separate tracing of payload
and header sending/receipt.

In the send path for headers that have already been sent, we also
explicitly initialize the handle so it can be referenced by the later
tracepoint.

Signed-off-by: Andrew Hall <hall@fb.com>
Signed-off-by: Matt Mullins <mmullins@fb.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

nbd: trace sending nbd requests

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: Matt Mullins <mmullins@fb.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>