When a process is writing and another process is changing the
configuration, it may happen that the trigger-related store calling
abort_disable() zeroes the active_block that is already being pushed
by the other process.

This fixes the interaction by keeping the cset spinlock during the whole
configuration of a sysfs attribute, thus preventing a write to sneak in.

After changing parameters, if the trigger was armed, the code re-arms
it, and this covers self-timed devices.  The choice helps simple tests
from the shell, but serious users will likely want to manually disable
the trigger before reconfiguring it.

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