Btrfs: use the new helper wbc_to_write_flags

Btrfs: use the new helper wbc_to_write_flags

This updates btrfs to use the helper wbc_to_write_flags which has been
applied in ext4/xfs/f2fs/block.

Please note that, with this, btrfs's dirty pages written by a
writeback job will carry the flag REQ_BACKGROUND, which is currently
used by writeback-throttle to determine whether it should go to get a
request or wait.

Signed-off-by: Liu Bo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>