9pfs: fix qemu_mknodat(S_IFREG) on macOS

mknod() on macOS does not support creating regular files, so
divert to openat_file() if S_IFREG is passed with mode argument.

Furthermore, 'man 2 mknodat' on Linux says: "Zero file type is
equivalent to type S_IFREG".

Link: https://lore.kernel.org/qemu-devel/17933734.zYzKuhC07K@silver/
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Will Cohen <wwcohen@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <3102ca936f88bc1f79d2a325e5bc68f48f54e6e3.1651228000.git.qemu_oss@crudebyte.com>

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

@@ -77,6 +77,15 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, const char *name,
 int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
 {
     int preserved_errno, err;
+
+    if (S_ISREG(mode) || !(mode & S_IFMT)) {
+        int fd = openat_file(dirfd, filename, O_CREAT, mode);
+        if (fd == -1) {
+            return fd;
+        }
+        close(fd);
+        return 0;
+    }
     if (!pthread_fchdir_np) {
         error_report_once("pthread_fchdir_np() not available on this version of macOS");
         return -ENOTSUP;