Merge patch series "fs: tweak mntns iteration"

Christian Brauner <brauner@kernel.org> says:

Make finding the last or first mount to start iterating the mount
namespace from an O(1) operation and add selftests for iterating the
mount table starting from the first and last mount.

* patches from https://lore.kernel.org/r/20241215-vfs-6-14-mount-work-v1-0-fd55922c4af8@kernel.org:
  selftests: add listmount() iteration tests
  fs: cache first and last mount
  fs: kill MNT_ONRB

Link: https://lore.kernel.org/r/20241215-vfs-6-14-mount-work-v1-0-fd55922c4af8@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>