mount: handle a case when parent and child mounts in the same directory

if mountpoints of ct and t are equal we can't build absolute path
in ct_mpnt_rpath, so let's skip the first "/" in m_root_rpath

v2: add a comment
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>