[msan] Fix SetShadow for mappings at the end of the application address space

[foad]

Summary

On PPC64 if you disable ASLR (or run under gdb) you're likely to see
mmap returning a mapping right at the end of the application address
space region. This caused SetShadow to call MEM_TO_SHADOW() on the
last+1 address in the region, which seems wrong to me; how can
MEM_TO_SHADOW() distinguish this from the first address in the following
region?

Fixed by only calling MEM_TO_SHADOW() once, on the start address.

[eugenis]

LGTM, good catch

lib/msan/msan_poisoning.cc

 void CopyMemory(void *dst, const void *src, uptr size, StackTrace *stack) {
   REAL(memcpy)(dst, src, size);
   CopyShadowAndOrigin(dst, src, size, stack);
 }
 
 void SetShadow(const void *ptr, uptr size, u8 value) {
   uptr PageSize = GetPageSizeCached();
   uptr shadow_beg = MEM_TO_SHADOW(ptr);
-  uptr shadow_end = MEM_TO_SHADOW((uptr)ptr + size);
+  uptr shadow_end = shadow_beg + size;
   if (value ||
       shadow_end - shadow_beg < common_flags()->clear_shadow_mmap_threshold) {
     REAL(memset)((void *)shadow_beg, value, shadow_end - shadow_beg);
   } else {
     uptr page_beg = RoundUpTo(shadow_beg, PageSize);
     uptr page_end = RoundDownTo(shadow_end, PageSize);
 
     if (page_beg >= page_end) {