Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20211027' into staging

Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20211027' into staging

9pfs: performance fix and cleanup

* First patch fixes suboptimal I/O performance on guest due to previously
  incorrect block size being transmitted to 9p client.

* Subsequent patches are cleanup ones intended to reduce code complexity.

* remotes/cschoenebeck/tags/pull-9p-20211027:
  9pfs: use P9Array in v9fs_walk()
  9pfs: make V9fsPath usable via P9Array API
  9pfs: make V9fsString usable via P9Array API
  fsdev/p9array.h: check scalar type in P9ARRAY_NEW()
  9pfs: introduce P9Array
  9pfs: simplify blksize_to_iounit()
  9pfs: deduplicate iounit code
  9pfs: fix wrong I/O block size in Rgetattr

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>