__attach_mnt(): lose the second argument

It's always ->mnt_parent of the first one.  What the function does is
making a mount (with already set parent and mountpoint) visible - in
mount hash and in the parent's list of children.

IOW, it takes the existing rootwards linkage and sets the matching
crownwards linkage.

Renamed to make_visible(), while we are at it.

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