[scudo] Fix reallocate for MTE. (#190086) For MTE, we can't use the whole size or we might trigger a segfault. Therefore, use the exact size when MTE is enabled or the exact usable size parameter is true. Also, optimize out the call to getUsableSize and use a simpler calculation. GitOrigin-RevId: 7c260d3966534031f9fe3a35b54887f6d979606d