selftests/bpf: Test RENAME_EXCHANGE and RENAME_NOREPLACE on bpffs

Add tests to exercise the behaviour of RENAME_EXCHANGE and RENAME_NOREPLACE
on bpffs. The former checks that after an exchange the inode of two
directories has changed. The latter checks that the source still exists
after a failed rename. Generally, having support for renameat2(RENAME_EXCHANGE)
in bpffs fixes atomic upgrades of our sk_lookup control plane.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211028094724.59043-5-lmb@cloudflare.com