thp: Introduce arch_(un)block_thp_split

In kernel/futex.c an assumption is made in the THP tail pinning code
that disabling the irqs is sufficient to block the THP splitting code
from completing.

Architectures such as ARM do hardware TLB broadcasts so disabling the
irqs will not prevent a THP split from completing.

This patch introduces: arch_block_thp_split and arch_unblock_thp_split,
and provides stock implementations that observe the original irq enable
and disable behaviour.

One can then define these macros themselves to implement architecture
specific behaviour.

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Kim Phillips <kim.phillips@linaro.org>

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