tty/vt: consolemap: extract dict unsharing to con_unshare_unimap()

The code in con_set_unimap() is too nested. Extract its obvious part
into a separate function and name it after what the code does:
con_unshare_unimap().

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220607104946.18710-19-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>