fs: improve readFile performance

This increases the maximum buffer size per read to 512kb when using
`fs.readFile`. This is important to improve the read performance for
bigger files.

PR-URL: https://github.com/nodejs/node/pull/27063
Refs: https://github.com/nodejs/node/issues/25741


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jamie Davis <davisjam@vt.edu>