Merge patch series "fs: lockless mntns lookup"

Christian Brauner <brauner@kernel.org> says:

Currently we take the read lock when looking for a mount namespace to
list mounts in. We can make this lockless. The simple search case can
just use a sequence counter to detect concurrent changes to the rbtree.

For walking the list of mount namespaces sequentially via nsfs we keep a
separate rcu list as rb_prev() and rb_next() aren't usable safely with
rcu.

Since creating mount namespaces is a relatively rare event compared with
querying mounts in a foreign mount namespace this is worth it. Once
libmount and systemd pick up this mechanism to list mounts in foreign
mount namespaces this will be used very frequently.

* patches from https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-0-6e3cdaf9b280@kernel.org:
  samples: add test-list-all-mounts
  selftests: remove unneeded include
  selftests: add tests for mntns iteration
  seltests: move nsfs into filesystems subfolder
  fs: simplify rwlock to spinlock
  fs: lockless mntns lookup for nsfs
  rculist: add list_bidir_{del,prev}_rcu()
  fs: lockless mntns rbtree lookup
  fs: add mount namespace to rbtree late
  mount: remove inlude/nospec.h include

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-0-6e3cdaf9b280@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>