[RFC] remove implicit slab.h inclusion from percpu.h

[Tejun Heo]

Hello,

Lee Schermerhorn was trying to use percpu from slab.h and ran into a
dependency loop.  percpu.h was using slab.h for UP inline
implementation which isn't a big deal in itself but it turns out that
percpu.h ends up being included everywhere via module.h and sched.h.
So, removing that implicit inclusion breaks a lot of files.

The following git tree contains trial conversion on x86_64.
allmodconfig builds fine on it but a lot of other archs are likely to
break although fixing them up shouldn't be too hard.

  git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu.git review-remove-implied-slab-inclusion

This can be solved much easier by moving some of the stuff that's
necessary for slab.h from percpu.h into percpu-defs.h which originally
got separated so that it can be used by asm/percpu.h but it's hackish
and for longer term, it would be better to have slab.h explicitly
included where necessary.

So, what do you guys think?  Probably-the-right-thing-to-do >544 file
patch or somewhat-ugly-but-let's-worry-about-it-tommorrow two file
patch?

I omitted the patch itself as it's mind numbingly boring and long.

Thanks.

Patch description and diffstat follows.  Patch content omitted.
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