From: Pasha Tatashin <redacted>
Date: Thu, 11 May 2017 16:59:33 -0400

We should either keep memset() only for deferred struct pages as what
I have in my patches.

Another option is to add a new function struct_page_clear() which
would default to memset() and to something else on platforms that
decide to optimize it.

On SPARC it would call STBIs, and we would do one membar call after
all "struct pages" are initialized.

No membars will be performed for single individual page struct clear,
the cutoff to use the STBI is larger than that.