fs: simplify FSReqBase slightly

Replace the `data_` member of `FSReqBase` by a boolean flag, because
the pointer that `data()` returns is already available through the
`buffer_` member, and set the default size for not requiring an
extra allocation to a more reasonable value, as the input is
usually something like a file path.

PR-URL: https://github.com/nodejs/node/pull/19174


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>