9pfs: fix qemu_mknodat() to always return -1 on error on macOS host

qemu_mknodat() is expected to behave according to its POSIX API, and
therefore should always return exactly -1 on any error, and errno
should be set for the actual error code.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <c714b5e1cae225ab7575242c45ee0fe4945eb6ad.1651228001.git.qemu_oss@crudebyte.com>

hw/9pfs/9p-util-darwin.c

@@ -124,7 +124,8 @@ int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
     }
     if (!pthread_fchdir_np) {
         error_report_once("pthread_fchdir_np() not available on this version of macOS");
-        return -ENOTSUP;
+        errno = ENOTSUP;
+        return -1;
     }
     if (pthread_fchdir_np(dirfd) < 0) {
         return -1;