work around linux not honoring write_at for O_APPEND files

[the8472]

the workaround only applies on kernels >= 6.9

fixes #140867

[rustbot]

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[rust-log-analyzer]

The job mingw-check-2 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- fs::tests::file_test_append_write_at stdout ----

thread 'fs::tests::file_test_append_write_at' panicked at library/std/src/fs/tests.rs:507:5:
assertion `left == right` failed
  left: [105, 116, 39, 115, 32, 110, 111, 116, 32, 119, 111, 114, 107, 105, 110, 103, 33, 32, 32, 32]
 right: [105, 116, 39, 115, 32, 32, 32, 32, 32, 119, 111, 114, 107, 105, 110, 103, 33]

---- os::unix::fs::tests::write_vectored_at stdout ----

thread 'os::unix::fs::tests::write_vectored_at' panicked at library/std/src/os/unix/fs/tests.rs:57:5:
assertion `left == right` failed
  left: [112, 119, 114, 105, 116, 101, 118, 32, 105, 115, 32, 110, 111, 116, 32, 119, 111, 114, 107, 105, 110, 103, 33, 32, 32, 32, 32, 103, 114, 101, 97, 116, 32, 32]
 right: [112, 119, 114, 105, 116, 101, 118, 32, 105, 115, 32, 32, 32, 32, 32, 103, 114, 101, 97, 116, 32, 32, 33]

[the8472]

Weird, the kernel should be new enough.