Description: nbd: freeze the queue while we're adding connections
References:
 https://lore.kernel.org/linux-block/24dff677353e2e30a71d8b66c4dffdbdf77c4dbd.1611595239.git.josef@toxicpanda.com/
 https://www.openwall.com/lists/oss-security/2021/01/28/3
Notes:
Bugs:
upstream: released (5.11-rc6) [b98e762e3d71e893b221f871825dc64694cfb258]
5.10-upstream-stable: released (5.10.13) [41f6f4a3143506ea1499cda2f14a16a2f82118a8]
4.19-upstream-stable: released (4.19.173) [424838c0f727f1d11ce2ccaabba96f4346c03906]
4.9-upstream-stable: N/A "Vulnerable code not present"
sid: released (5.10.13-1)
4.19-buster-security: released (4.19.177-1)
4.9-stretch-security: N/A "Vulnerable code not present"