nbd: clear queue limits on disconnect

An inactive nbd device may refuse any I/O operations. The nbd_config_put
function calls invalidate_disk, which sets the device capacity to zero
to reject all read and write I/O. For zero-sector flush I/O requests
from blkdev_issue_flush, if the write cache is disabled, the zero-sector
flush I/O immediately returns 0 in submit_bio_noacct. However, since
nbd_config_put does not clear the write cache state, an inactive nbd
device might still have the write cache enabled. In this situation,
zero-sector flush I/O will return -EIO because there is no active socket.
Additionally, BLK_FEAT_FUA and BLK_FEAT_ROTATIONAL flags may also remain
stale, resetting all of them ensures consistent behavior.

The limits update uses queue_limits_commit_update() (the non-freezing
variant) because config_refs == 0 here means every fd is closed and recv
threads have drained, so no in-flight I/O can read q->limits concurrently.

Reviewed-by: Yu Kuai <yukuai@fygo.io>
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Link: https://patch.msgid.link/20260805122930.57647-4-yangerkun@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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