tty: Reserve ldisc 29 for development purposes

It's handy to have an ldisc number free for out-of-tree testing. This
way, a new ldisc can be developed on any running system, without having
to recompile the kernel just to define a new number.

This is the highest number (and also the last one) available under the
old numbering scheme, so let's reserve it before it's too late.

From now on, every new ldisc upstreamed will have to increment NR_LDISCS
in lockstep with its addition to the table in tty.h.

Signed-off-by: Max Staudt <max@enpas.org>
Link: https://lore.kernel.org/r/20220211141036.6403-1-max@enpas.org


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