new helper: traverse_mounts()

common guts of follow_down() and follow_managed() taken to a new
helper - traverse_mounts().  The remnants of follow_managed()
are folded into its sole remaining caller (handle_mounts()).
Calling conventions of handle_mounts() slightly sanitized -
instead of the weird "1 for success, -E... for failure" that used
to be imposed by the calling conventions of walk_component() et.al.
we can use the normal "0 for success, -E... for failure".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>