Merge remote-tracking branch 'remotes/gkurz/tags/for-upstream' into staging

Merge remote-tracking branch 'remotes/gkurz/tags/for-upstream' into staging

Fixes yet another use-after-free issue that could be triggered by a
misbehaving guest. This is a follow-up to commit:

commit 5b76ef50f62079a2389ba28cacaf6cce68b1a0ed
Author: Greg Kurz <groug@kaod.org>
Date:   Wed Nov 7 01:00:04 2018 +0100

    9p: write lock path in v9fs_co_open2()

# gpg: Signature made Tue 20 Nov 2018 12:01:07 GMT
# gpg:                using RSA key 71D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <groug@kaod.org>"
# gpg:                 aka "Gregory Kurz <gregory.kurz@free.fr>"
# gpg:                 aka "[jpeg image of size 3330]"
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/for-upstream:
  9p: take write lock on fid path updates (CVE-2018-19364)

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