[PATCH 0/2] completion: Reduce stack usage caused by COMPLETION_INITIALIZER_ONSTACK()

[Boqun Feng]

With LOCKDEP_CROSSRELEASE and LOCKDEP_COMPLETIONS introduced, the growth
in kernel stack usage of several functions were reported:

	https://marc.info/?l=linux-kernel&m=150270063231284&w=2

The root cause of this is in COMPLETION_INITIALIZER_ONSTACK(), we use

	({init_completion(&work); work})

, which will create a temporary object when returned. However this
temporary object is unnecessary. And this patch fixes it by making the
statement expression in COMPLETION_INITIALIZER_ONSTACK() return a
pointer rather than a whole structure. This will reduce the stack usage
even if !LOCKDEP.

However, such a change does make one COMPLETION_INITIALIZER_ONSTACK()
callsite invalid, so we fix this first via converting to
init_completion().

Regards,
Boqun