qemu-nbd: regression with arguments passing into nbd_client_thread()

qemu-nbd: regression with arguments passing into nbd_client_thread()

Unfortunately
    commit 03b67621445d601c9cdc7dfe25812e9f19b81488
    (7.2:  6e216d21b56a7545a05080a370b5ca7491fecfb3)
    Author: Denis V. Lunev <den@openvz.org>
    Date:   Mon Jul 17 16:55:40 2023 +0200
    qemu-nbd: pass structure into nbd_client_thread instead of plain char*
has introduced a regression. struct NbdClientOpts resides on stack inside
'if' block. This specifically means that this stack space could be reused
once the execution will leave that block of the code.

This means that parameters passed into nbd_client_thread could be
overwritten at any moment.

The patch moves the data to the namespace of main() function effectively
preserving it for the whole process lifetime.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: <qemu-stable@nongnu.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230727105828.324314-1-den@openvz.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit e5b815b0defcc3617f473ba70c3e675ef0ee69c2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: add reference to 6e216d21b56a7545a05080a370b5ca7491fecfb3 for 7.2 branch)