arm: mm: enable RCU fast_gup

arm: mm: enable RCU fast_gup

Activate the RCU fast_gup for ARM.  We also need to force THP splits to
broadcast an IPI s.t.  we block in the fast_gup page walker.  As THP
splits are comparatively rare, this should not lead to a noticeable
performance degradation.

Some pre-requisite functions pud_write and pud_page are also added.

Signed-off-by: Steve Capper <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Cc: Dann Frazier <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Russell King <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Christoffer Dall <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>