mm: extract might_alloc() debug check

Extracted from slab.h, which seems to have the most complete version
including the correct might_sleep() check.  Roll it out to slob.c.

Motivated by a discussion with Paul about possibly changing call_rcu
behaviour to allocate memory, but only roughly every 500th call.

There are a lot fewer places in the kernel that care about whether
allocating memory is allowed or not (due to deadlocks with reclaim code)
than places that care whether sleeping is allowed.  But debugging these
also tends to be a lot harder, so nice descriptive checks could come in
handy.  I might have some use eventually for annotations in drivers/gpu.

Note that unlike fs_reclaim_acquire/release gfpflags_allow_blocking does
not consult the PF_MEMALLOC flags.  But there is no flag equivalent for
GFP_NOWAIT, hence this check can't go wrong due to
memalloc_no*_save/restore contexts.  Willy is working on a patch series
which might change this:

https://lore.kernel.org/linux-mm/[email protected]/

I think best would be if that updates gfpflags_allow_blocking(), since
there's a ton of callers all over the place for that already.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Qian Cai <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Christian König <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Thomas Hellström (Intel) <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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