fs: lockless mntns lookup for nsfs

fs: lockless mntns lookup for nsfs

We already made the rbtree lookup lockless for the simple lookup case.
However, walking the list of mount namespaces via nsfs still happens
with taking the read lock blocking concurrent additions of new mount
namespaces pointlessly. Plus, such additions are rare anyway so allow
lockless lookup of the previous and next mount namespace by keeping a
separate list. This also allows to make some things simpler in the code.

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-5-6e3cdaf9b280@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>