Add support for renameat2(RENAME_EXCHANGE) on bpffs. This is useful
for atomic upgrades of our sk_lookup control plane.

* Create a temporary directory on bpffs
* Migrate maps and pin them into temporary directory
* Load new sk_lookup BPF, attach it and pin the link into temp dir
* renameat2(temp dir, state dir, RENAME_EXCHANGE)
* rmdir temp dir

Due to the sk_lookup semantics this means we can never end up in a
situation where an upgrade breaks the existing control plane.

Lorenz Bauer (2):
  libfs: support RENAME_EXCHANGE in simple_rename()
  selftests: bpf: test RENAME_EXCHANGE and RENAME_NOREPLACE on bpffs

 fs/libfs.c                                    |  6 ++-
 .../selftests/bpf/prog_tests/test_bpffs.c     | 39 +++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

-- 
2.30.2