[scudo] For a realloc that shrinks, retag the extra. (#204031)

When MTE is enabled and an allocations is reallocated from a large size
to a smaller size, zero tag the rest of the allocation. Before this
change only a single granule after the new size was zero tagged. This
adds extra security and use after realloc protection if code would have
tried to read/write into the old size, past the new size.

GitOrigin-RevId: 5639b33d7567bf648f27418b126f5f6f6292b730