yes: avoid ambiguous PAGE_SIZE

[oech3]

This is actually same with page size. But this is used to checking that partial tee is allowed. So explicitly use The maximum length at which writes to a pipe are atomic.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[xtqqczze]

• GnuTests / Aggregate GNU test results (pull_request) failure tracked by GNU: tests/timeout/timeout-group intermittent failure #12354.

[sylvestre]

LGTM — PIPE_BUF is semantically correct here (atomic write guarantee).