linux: add trivial renameat2 implementation

Just return EINVAL if flags != 0. The Linux man page documents one
case of EINVAL as "The filesystem does not support one of the flags in
flags."

After r351723 userland binaries will try using new system calls.

Reported by: mjg
Reviewed by: mjg, trasz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21590