tty: extract tty_flip_buffer_commit() from tty_flip_buffer_push()

commit 716b1058

 upstream.

We will need this new helper in the next patch.

Cc: Hillf Danton <hdanton@sina.com>
Cc: 一只狗 <chennbnbnb@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220707082558.9250-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>