mm: introduce a general RCU get_user_pages_fast()

This series implements general forms of get_user_pages_fast and
__get_user_pages_fast in core code and activates them for arm and arm64.

These are required for Transparent HugePages to function correctly, as a
futex on a THP tail will otherwise result in an infinite loop (due to the
core implementation of __get_user_pages_fast always returning 0).

Unfortunately, a futex on THP tail can be quite common for certain
workloads; thus THP is unreliable without a __get_user_pages_fast
implementation.

This series may also be beneficial for direct-IO heavy workloads and
certain KVM workloads.

This patch (of 6):

get_user_pages_fast() attempts to pin user pages by walking the page
tables directly and avoids taking locks.  Thus the walker needs to be
protected from page table pages being freed from under it, and needs to
block any THP splits.

One way to achieve this is to have the walker disable interrupts, and rely
on IPIs from the TLB flushing code blocking be...