[PATCH v2] Support for write stream IDs

[Jens Axboe]

Hi,

Changes since the last posting:

- Added a specific per-file fadvise setting. POSIX_FADV_STREAMID sets
  the inode and file stream ID, POSIX_FADV_STREAMID_FILE sets just the
  file stream ID.

- Addressed review comments.

I've since run some testing with write streams. Test case was a RocksDB
overwrite benchmark, using 3 billion keys of 400B in size (numbers set
use the full size of the device). WAL/LOG was assigned to stream 1, and
each RocksDB compaction level used a separate stream. With streams
enabled, user write to device writes (write amplification) was at 2.33.
Without streams, the write amplification was 3.05. That is roughly 20%
less written NAND, and the streams test subsequently also had 20%
higher throughput.

Unless there are any grave concerns here, I'd like to merge this for
4.2.

-- 
Jens Axboe

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