fuse: add fuse_request_sent tracepoint

fuse: add fuse_request_sent tracepoint

This new tracepoint complements fuse_request_send (enqueue) and
fuse_request_end (completion).  It fires after the request has been
successfully copied to the daemon's buffer, just before the daemon
can start to process it.

fuse_request_sent does not fire if the copy of the request fails.
It also does not fire for NOTIFY_REPLY, which fires the _end tracepoint
at the end of copy.

This is needed for tools tracking the in-flight state of user initiated
fuse requests.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>