If a cset->flags has ZIO_CSET_INTERLEAVE_ONLY or
ZIO_CSET_CHAN_INTERLEAVE, then ZIO creates an additional channel in
the cset: the interleaved channel.  As for normal channels, the
interleaved channel has 2 char device:

/dev/zio/devname-0-i-ctrl
/dev/zio/devname-0-i-data

If cset flags include ZIO_CSET_INTERLEAVE_ONLY, ZIO creates only
interleaved channel char devices. From the sysfs you can
enable/disable only the interleaved channel; all the other channels
are always disabled.

If cset flags include ZIO_CSET_CHAN_INTERLEAVE, ZIO creates all char
devices for normal channels and also for the interleaved channel. By
default the interleaved channel is disabled. You can enable the
interleaved channel from sysfs; this choice automatically disable all
the other channels.

Interleave support is needed for the fmc-adc-100ms-14b-4cha driver.
Maybe this is not the final approach to interleave, but it is
working and well-tested, so the time is ripe to have it in master.

Maybe we'll change some details about interleave support, in the
future.
Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>