LinuxKPI: tasklet add (*callback) support

[bz]

Summary

In addition to (*func) tasklet also seems to have a (*callback) with
a different argument. Add support for this and add tasklet_setup()
as well for support in more drivers.

The from_tasklet() definition is duplicated in the tree; hide it there
under #ifndef to avoid a re-definition. People should generally add
LinuxKPI bits to linuxkpi rather than private files if they also rely
on other LinuxKPI bits.

X-MFC: DO NOT MFC, space allocated by drivers not us.

[bz]

It's been months since I had done this change and I wonder if we could do it without the bool and just check if func is != NULL then use func and otherwise use callback; that would also make it MFCable I think. Someone might have to take a peek at Linux and check if there is the invariant of only func or callback that can be set at one time and whether one has to be set?

[emaste]

if there is the invariant of only func or callback that can be set at one time

This does not appear to be the case.

[bz]

In D37216#845151, @emaste wrote:

if there is the invariant of only func or callback that can be set at one time

This does not appear to be the case.

Thanks Ed! So we need the bool and we cannot MFC (correct me if I misunderstand).

[bz]

Anyone up for the review?