btrfs: pass write-hint for buffered IO

Commit 449813515d3e ("block, fs: Restore the per-bio/request data
lifetime fields") restored write-hint support in btrfs. But that is
applicable only for direct IO. This patch supports passing
write-hint for buffered IO from btrfs file system to block layer
by filling bi_write_hint of struct bio in alloc_new_bio().

There's an ongoing discussion which devices can use that,
https://lore.kernel.org/all/[email protected],
in SCSI there's support using sd_group_number() and
sd_setup_rw32_cmnd().

The hint goes from the application directly to the block device so it's
up to the application to set up everything properly to utilize the
different hint classes.

Link: https://lore.kernel.org/all/[email protected]
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Jing Xia <[email protected]>
[ Add more context and use case. ]
Signed-off-by: David Sterba <[email protected]>

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