namei: move cross-device check to __traverse_mounts

This is preparation to RESOLVE_NO_XDEV fix in following commits.
Also this commit makes LOOKUP_NO_XDEV logic more clear: now we
immediately fail with EXDEV on first mount crossing
instead of waiting for very end.

No functional change intended

Signed-off-by: Askar Safin <safinaskar@zohomail.com>
Link: https://lore.kernel.org/20250825181233.2464822-4-safinaskar@zohomail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>