arm64: pgtable: Clear the GP bit for non-executable kernel pages

arm64: pgtable: Clear the GP bit for non-executable kernel pages

Commit cca98e9f8b5e ("mm: enforce that vmap can't map pages executable")
introduced 'pgprot_nx(prot)' for arm64 but collided silently with the
BTI support during the merge window, which endeavours to clear the GP
bit for non-executable kernel mappings in set_memory_nx().

For consistency between the two APIs, clear the GP bit in pgprot_nx().

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200615154642.3579-1-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>