arm64: insn: avoid virt_to_page() translations on core kernel symbols

Before restricting virt_to_page() to the linear mapping, ensure that
the text patching code does not use it to resolve references into the
core kernel text, which is mapped in the vmalloc area.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>