docs: path-lookup: update follow_managed() part

No follow_managed() anymore, handle_mounts(),
traverse_mounts(), will do the job.
see commit 9deed3eb ("new helper: traverse_mounts()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-2-foxhlchen@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>