include/linux/zio-sysfs.h: introduce helper to retrieve std attributes

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
trigger: new trigger operation arm to handle trigger arming

This patch add a new trigger operation: arm().
The operation is useful because it allows a trigger to define its own
procedure to arm itself.

At the moment, nobody use the trigger operations data_done() and abort()
for custom tasks because the generic one works in most of the case.
Tipically there is nothing special to do if the arming procedure is
always the same.
This patch make sense of these operations because if you need a custom
arm(), probably, you also need a custom data_done() and abort().

There is no use of the arm() operation in the ZIO repository, but there
are some devices outside this repository that need this operation

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

Add to zio_device a new operation: change_flags. The core uses this
function to signal the driver that some flags are changed.

This patch also improve the comment on the __zobj_enable function

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