nbd: rename the nbd_device variable from lo to nbd

rename the nbd_device variable from "lo" to "nbd", since "lo" is just a name
copied from loop.c.
Signed-off-by: NWanlong Gao <gaowanlong@cn.fujitsu.com>
Cc: Paul Clements <paul.clements@steeleye.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>