FROMLIST: 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.

Bug: 145210207
Change-Id: I9fbf40769c5c241422fff8558c7a9bade8ebadb6
(am from https://lore.kernel.org/patchwork/patch/1149052/

)
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>