arm64/mm: remove unnecessary boundary check

Remove the unnecessary boundary check since there is a huge
gap between user and kernel address that they would never overlap.
(arm64 does not have enough levels of page tables to cover 64-bit
virtual address)

See Documentation/arm64/memory.txt

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>