target/arm:Set lg_page_size to 0 if either S1 or S2 asks for it

In get_phys_addr_twostage() we set the lg_page_size of the result to
the maximum of the stage 1 and stage 2 page sizes.  This works for
the case where we do want to create a TLB entry, because we know the
common TLB code only creates entries of the TARGET_PAGE_SIZE and
asking for a size larger than that only means that invalidations
invalidate the whole larger area.  However, if lg_page_size is
smaller than TARGET_PAGE_SIZE this effectively means "don't create a
TLB entry"; in this case if either S1 or S2 said "this covers less
than a page and can't go in a TLB" then the final result also should
be marked that way.  Set the resulting page size to 0 if either
stage asked for a less-than-a-page entry, and expand the comment
to explain what's going on.

This has no effect for VMSA because currently the VMSA lookup always
returns results that cover at least TARGET_PAGE_SIZE; however when we
add v8R support it will reuse this code path, and for v8R the S1 and
S2 results can be smaller than TARGET_PAGE_SIZE.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20221212142708.610090-1-peter.maydell@linaro.org

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