Merge patch series "pidfs: support bind-mounts"

Christian Brauner <brauner@kernel.org> says:

Allow bind-mounting pidfds. Similar to nsfs let's allow bind-mounts for
pidfds. This allows pidfds to be safely recovered and checked for
process recycling.

Instead of checking d_ops for both nsfs and pidfs we could in a
follow-up patch add a flag argument to struct dentry_operations that
functions similar to file_operations->fop_flags.

* patches from https://lore.kernel.org/r/20241219-work-pidfs-mount-v1-0-dbc56198b839@kernel.org:
  selftests: add pidfd bind-mount tests
  pidfs: allow bind-mounts

Link: https://lore.kernel.org/r/20241219-work-pidfs-mount-v1-0-dbc56198b839@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>