nbd: Rename functions for clearness of recv/send path

This patch renames functions so that it is clear what the function does.
Otherwise it is not directly understandable what for example 'do_it' means.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Jens Axboe <axboe@fb.com>