target-i386: do not duplicate page protection checks

x86_cpu_handle_mmu_fault is currently checking twice for writability
and executability of pages; the first time to decide whether to
trigger a page fault, the second time to compute the "prot" argument
to tlb_set_page_with_attrs.

Reorganize code so that first "prot" is computed, then it is used
to check whether to raise a page fault, then finally PROT_WRITE is
removed if the D bit will have to be set.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
