zio-trigger.h: offer zio_generic_data_done() as inline function

The function used to be called __zio_internal_data_done, in
heklpers.c.  Being used once only, it was expanded inline by the
compiler, so nothing changes for previous code.

However, if a trigger needs to implement its own data_done method, it
will most likely need to replicate this code, so it's better made
avaiable directly to everybody.  An alternative would be running this
code every time, before or after calling t_op->data_done, but we'd
better let each trigger choose the order of its own operations.

Actually, there is one difference: the input active block is set to NULL
after being stored to the current buffer, so the later implementation
of stop_io (used in zio-irq-tdc.c) won't expose a bug.

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