nbd/client: refactor TRACE of NBD_MAGIC

We are going to switch from TRACE macro to trace points,
this TRACE complicates things, this patch simplifies it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170707152918.23086-7-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>