fs: align fs.ReadStream buffer pool writes to 8-byte boundary

Prevents alignment issues when creating a typed array from a buffer.

Fixes: https://github.com/nodejs/node/issues/24817

PR-URL: https://github.com/nodejs/node/pull/24838


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>