attach_recursive_mnt(): unify the mnt_change_mountpoint() logics

attach_recursive_mnt(): unify the mnt_change_mountpoint() logics

The logics used for tucking under existing mount differs for original
and copies; copies do a mount hash lookup to see if mountpoint to be is
already overmounted, while the original is told explicitly.

But the same logics that is used for copies works for the original,
at which point the only place where we get very close to eliminating
the need of passing 'beneath' flag to attach_recursive_mnt().

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