mount: return a relative path from cut_root_for_bind()

static char *cut_root_for_bind(char *target_root, char *source_root)

Currently it returns a relative path if source_root is '/' and
an absolute path for other cases.

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