nbd: remove define-only NBD_MAGIC, previously magic number

commit f4507164 ("nbd: rename the nbd_device variable from lo to
nbd") renamed LO_MAGIC to NBD_MAGIC; commit 5ea8d108 ("nbd:
separate out the config information") removed the last users of that

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/10a80681c5966fed1a1afc696e3db114f481514c.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>