arm64: mm: fix thinko in non-global page table attribute check

The routine pgattr_change_is_safe() was extended in commit 4e602056
("arm64: mm: Permit transitioning from Global to Non-Global without BBM")
to permit changing the nG attribute from not set to set, but did so in a
way that inadvertently disallows such changes if other permitted attribute
changes take place at the same time. So update the code to take this into
account.

Fixes: 4e602056 ("arm64: mm: Permit transitioning from Global to ...")
Cc: <stable@vger.kernel.org> # 4.14.x-
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>