sort: update DEFAULT_BUF_SIZE to 8 KiB

[xtqqczze]

https://codspeed.io/uutils/coreutils/branches/xtqqczze%3ADEFAULT_BUF_SIZE

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/date/date-locale-hour is no longer failing!

[oech3]

just wait few time. There is some delay.

[oech3]

looks no problem https://codspeed.io/uutils/coreutils/branches/xtqqczze%3ADEFAULT_BUF_SIZE?utm_source=github&utm_medium=check&utm_content=button

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/date/date-locale-hour is no longer failing!

[xtqqczze]

No benchmark diffs, I guess the allocator generally rounds up to next power of 2.

[cakebaker]

Thanks!