Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2021-04-13' into staging

Block patches for 6.0-rc3:
- Use-after-free fix for block/nbd.c

# gpg: Signature made Tue 13 Apr 2021 14:35:48 BST
# gpg:                using RSA key 91BEB60A30DB3E8857D11829F407DB0061D5CF40
# gpg:                issuer "mreitz@redhat.com"
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>" [full]
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2021-04-13:
  block/nbd: fix possible use after free of s->connect_thread

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>