Description: nbd: Fix NULL pointer in flush_workqueue
References:
Notes:
 carnil> Introduced in e9e006f5fcf2 ("nbd: fix max number of supported devs").
 carnil> Vulnerable versions: 4.14.149 4.14.161 4.19.79 4.19.92 5.3.6 5.4-rc1 5.4.7.
Bugs:
upstream: released (5.13-rc2) [79ebe9110fa458d58f1fceb078e2068d7ad37390]
6.7-upstream-stable: N/A "Fixed before branching point"
6.6-upstream-stable: N/A "Fixed before branching point"
6.1-upstream-stable: N/A "Fixed before branching point"
5.10-upstream-stable: released (5.10.38) [cde4b55cfb24522dcbba80bbdb0c082303e76c43]
4.19-upstream-stable: needed
sid: released (5.10.38-1)
6.1-bookworm-security: N/A "Fixed before branching point"
5.10-bullseye-security: N/A "Fixed before branching point"
4.19-buster-security: needed