nbd: lower the loglevel of an error message

This is only an error, no need to use KERN_CRIT log level.

Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Paul Clements <Paul.Clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>