Pass the length of a string to respond_string and use
tty_insert_flip_string instead of a loop with tty_insert_flip_char. This
simplifies the processing on the tty side.

The added strlens are optimized during constant folding and propagation
and the result are proper constants in assembly.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-8-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>