Makefile: replace EXTRA_CFLAGS with ccflags-y

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Makefile: add debug option

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-ldisc.c: set receive_room size

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-ldisc.c: add ldisc_instance to handle zdev

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-ldisc.c: each instance has its own buffer

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-ldisc.c: user chan->priv_d as block position

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>