arm64: mm: avoid x18 in idmap_kpti_install_ng_mappings

idmap_kpti_install_ng_mappings uses x18 as a temporary register, which
will result in a conflict when x18 is reserved. Use x16 and x17 instead
where needed.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>