[PATCH RFC v2] Support for write stream IDs

[Jens Axboe <axboe@fb.com>]

One of the things that exacerbates write amplification on flash
based devices is that fact that data with different lifetimes get
grouped together on media. Currently we have no interface that
applications can use to separate different types of writes. This
patch set adds support for that.

The kernel has no knowledge of what stream ID is what. The idea is
that writes with identical stream IDs have similar life times, not
that stream ID 'X' has a shorter lifetime than stream ID 'X+1'.

There are basically two interfaces that could be used for this. One
is fcntl, the other is fadvise. This patchset uses fadvise, with a
new POSIX_FADV_STREAMID hint. The 'offset' field is used to pass
the relevant stream ID. Switching to fcntl (with a SET/GET_STREAMID)
would be trivial.

The patchset wires up the block parts, adds buffered and O_DIRECT
support, and modifies btrfs/xfs too. It should be trivial to extend
this to all other file systems, I just used xfs and btrfs for testing.

No block drivers are wired up yet. Patches are against current -git.

Changes since v1:

- Don't add streamid to struct writeback_control, always use the one
  in the inode for buffered writeback
- Add file_streamid() helper. Returns file streamid, if set, otherwise
  it returns the inode streamid.
- Update btrfs/xfs for wbc->streamid change.
- Add streamid to ext4.
- Bump stream bits from 3 to 8, 255 streams are now supported.