Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20210705' into staging

9pfs: misc patches

* Add link to 9p developer docs.

* Fix runtime check whether client supplied relative path is the export
  root.

* Performance optimization of Twalk requests.

* Code cleanup.

# gpg: Signature made Mon 05 Jul 2021 12:13:34 BST
# gpg:                using RSA key 96D8D110CF7AF8084F88590134C2B58765A47395
# gpg:                issuer "qemu_oss@crudebyte.com"
# gpg: Good signature from "Christian Schoenebeck <qemu_oss@crudebyte.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: ECAB 1A45 4014 1413 BA38  4926 30DB 47C3 A012 D5F4
#      Subkey fingerprint: 96D8 D110 CF7A F808 4F88  5901 34C2 B587 65A4 7395

* remotes/cschoenebeck/tags/pull-9p-20210705:
  9pfs: reduce latency of Twalk
  9pfs: drop root_qid
  9pfs: replace not_same_qid() by same_stat_id()
  9pfs: drop fid_to_qid()
  9pfs: capture root stat
  9pfs: fix not_same_qid()
  9pfs: simplify v9fs_walk()
  9pfs: add link to 9p developer docs

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