nbd/rc: load nbd module explicitly

nbd/rc: load nbd module explicitly

After the commit "common/rc: avoid module load in _have_driver()",
_have_driver() no longer loads specified module. However, nbd test cases
and _have_nbd_netlink() function assume that the module is loaded by
calling _have_driver(). This causes test case failures and unexpected
skips. To fix them, load and unload modules explicitly in functions
_start_nbd_server*(), _stop_nbd_server*() and _have_nbd_netlink().

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>