If the trigger is enabled on creation, it may fire before the cset is
ready.  This happened to me with "modprobe zio-zero trigger=irq".
For this reason it must start disabled. Also, the create
method MUST set ti->cset before activating the trigger, because the
cset lock is needed to access the enable/disable flag.

The commit adds a WARN() if the field is not set after creation, then
it leaves it zeroed to ensure the system will crash, instead of having
a subtle race.

Also, add a pair of FIXME notes about the need to allow changing
a trigger type while leaving the new one disabled.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>