arm64: mm: avoid fixmap for early swapper_pg_dir updates

Early in the boot, when .rodata is still writable, we can poke
swapper_pg_dir entries directly, and there is no need to go through the
fixmap. After a future patch, we will enter the kernel with
swapper_pg_dir already active, and early swapper_pg_dir updates for
creating the fixmap page table hierarchy itself cannot go through the
fixmap for obvious reaons. So let's keep track of whether rodata is
writable, and update the descriptor directly in that case.

As the same reasoning applies to early KASAN init, make the function
noinstr as well.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-67-ardb+git@google.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>