Remove the POSIX_FADV_DONTNEED and POSIX_FADV_SEQUENTIAL calls
from the local backend. The DONTNEED calls cause severe spinlock
contention on parallel file systems (and any system with many
concurrent transfers), because each call triggers per-page cache
teardown under a global lock.
Observed on a 256-core system running rclone with 64 parallel
transfers over Lustre: 69% of all CPU cycles were spent in
kernel spinlock contention from the fadvise path, with effective
throughput well below hardware capability.
The kernel's own page reclaim (kswapd) handles eviction more
efficiently from a single context. Since rclone does not always
read files sequentially (e.g. multipart uploads rewind and
re-read blocks), FADV_SEQUENTIAL was also not reliably correct.
This is consistent with the non-Linux behavior (which never
called fadvise) and with restic's decision to remove identical
code (restic/restic#670).
Fixes#7886
7ca667d35d