os: correct bad PathError message from FileOpen with O_CREATE on Plan 9

On Plan 9, FileOpen with flag O_CREATE & ~O_TRUNC is done in two
steps.  First, syscall.Open is attempted, to avoid truncation when opening
an existing file.  If that fails because the file doesn't exist,
syscall.Create is used to create a new file.  If the Create fails,
for example because we are racing with another process to create a
ModeExclusive file, the PathError returned from FileOpen should reflect
the result of the Create, not the "does not exist" error from the initial
Open attempt.

Fixes #38540

Change-Id: I90c95a301de417ecdf79cd52748591edb1dbf528
Reviewed-on: https://go-review.googlesource.com/c/go/+/229099
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/os/file_plan9.go

@@ -112,10 +112,9 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	} else {
 		fd, e = syscall.Open(name, flag)
 		if IsNotExist(e) && create {
-			var e1 error
-			fd, e1 = syscall.Create(name, flag, syscallMode(perm))
-			if e1 == nil {
-				e = nil
+			fd, e = syscall.Create(name, flag, syscallMode(perm))
+			if e != nil {
+				return nil, &PathError{"create", name, e}
 			}
 		}
 	}