linuxulator: ignore AT_NO_AUTOMOUNT for all stat variants

Commit ff39d74aa99a ignored AT_NO_AUTOMOUNT for statx(), but did not
change fstat64() or newfstatat(), which also take an equivalent flags
argument. Add a linux_to_bsd_stat_flags() helper and use it in all
three places.

PR: 281526
Reviewed by: trasz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46711