syscall: ensure Mkdir(path) on Plan 9 fails if path exists
On Plan 9, the underlying create() syscall with DMDIR flag, which is
used to implement Mkdir, will fail silently if the path exists and
is not a directory. Work around this by checking for existence
first and rejecting Mkdir with error EEXIST if the path is found.
Fixes #23918
Change-Id: I439115662307923c9f498d3e7b1f32c6d205e1ad
Reviewed-on: https://go-review.googlesource.com/94777 Reviewed-by: David du Colombier <0intro@gmail.com>