tty/vt: consolemap: use con_allocate_new() in con_unshare_unimap()

The old->refcount is guaranteed to be > 1, so we can directly call
con_allocate_new() to make the code more obvious.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220607104946.18710-35-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>