target/ppc: Improve helper_dcbz for user-only

target/ppc: Improve helper_dcbz for user-only

Mark the reserve_addr check unlikely.  Use tlb_vaddr_to_host
instead of probe_write, relying on the memset itself to test
for page writability.  Use set/clear_helper_retaddr so that
we can properly unwind on segfault.

With this, a trivial loop around guest memset will no longer
spend nearly 25% of runtime within page_get_flags.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>