mm/gup: refactor and de-duplicate gup_fast() code

There were two nearly identical sets of code for gup_fast() style of
walking the page tables with interrupts disabled.  This has lead to the
usual maintenance problems that arise from having duplicated code.

There is already a core internal routine in gup.c for gup_fast(), so just
enhance it very slightly: allow skipping the fall-back to "slow" (regular)
get_user_pages(), via the new FOLL_FAST_ONLY flag.  Then, just call
internal_get_user_pages_fast() from __get_user_pages_fast(), and adjust
the API to match pre-existing API behavior.

There is a change in behavior from this refactoring: the nested form of
interrupt disabling is used in all gup_fast() variants now.  That's
because there is only one place that interrupt disabling for page walking
is done, and so the safer form is required.  This should, if anything,
eliminate possible (rare) bugs, because the non-nested form of enabling
interrupts was fragile at best.

[[email protected]: fixup]
  Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: "Joonas Lahtinen" <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Souptick Joarder <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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