fs: add vfs_open_tree() helper

Split out vfs_open_tree() from open_tree() so we can use it in later
patches.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-1-c25feb0d2eb3@kernel.org


Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>