CVE-2025-68372: nbd: defer config put in recv_work

In the Linux kernel, the following vulnerability has been resolved:

nbd: defer config put in recvwork

There is one uaf issue in recvwork when running NBDCLEARSOCK and NBDCMDRECONFIGURE: nbdgenlconnect // confref=2 (connect and recvwork A) nbdopen // confref=3 recvwork A done // confref=2 NBDCLEARSOCK // confref=1 nbdgenlreconfigure // confref=2 (trigger recvwork B) close nbd // confref=1 recvwork B configput // confref=0 atomicdec(&config->recvthreads); -> UAF

Or only running NBDCLEARSOCK: nbdgenlconnect // confref=2 nbdopen // confref=3 NBDCLEARSOCK // confref=2 close nbd nbdrelease configput // confref=1 recvwork configput // confref=0 atomicdec(&config->recvthreads); -> UAF

Commit 87aac3a80af5 ("nbd: call nbdconfigput() before notifying the waiter") moved nbdconfigput() to run before waking up the waiter in recvwork, in order to ensure that nbdstartdeviceioctl() would not be woken up while nbd->taskrecv was still uncleared.

However, in nbdstartdeviceioctl(), after being woken up it explicitly calls flushworkqueue() to make sure all current works are finished. Therefore, there is no need to move the config put ahead of the wakeup.

Move nbdconfigput() to the end of recvwork, so that the reference is held for the whole lifetime of the worker thread. This makes sure the config cannot be freed while recvwork is still running, even if clear + reconfigure interleave.

In addition, we don't need to worry about recvwork dropping the last nbdput (which causes deadlock):

path A (netlink with NBDCFLAGDESTROYONDISCONNECT): connect // nbdrefs=1 (trigger recvwork) open nbd // nbdrefs=2 NBDCLEARSOCK close nbd nbdrelease nbddisconnectandput flushworkqueue // recvwork done nbdconfigput nbdput // nbdrefs=1 nbdput // nbdrefs=0 queuework

path B (netlink without NBDCFLAGDESTROYONDISCONNECT): connect // nbdrefs=2 (trigger recvwork) open nbd // nbdrefs=3 NBDCLEARSOCK // confrefs=2 close nbd nbdrelease nbdconfigput // confrefs=1 nbdput // nbdrefs=2 recvwork done // confrefs=0, nbdrefs=1 rmmod // nbdrefs=0

Depends-on: e2daec488c57 ("nbd: Fix hungtask when nbdconfigput")