arm64/mm: __always_inline to improve fork() perf

As set_ptes() and wrprotect_ptes() become a bit more complex, the
compiler may choose not to inline them. But this is critical for fork()
performance. So mark the functions, along with contpte_try_unfold()
which is called by them, as __always_inline. This is worth ~1% on the
fork() microbenchmark with order-0 folios (the common case).

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>