btrfs: introduce extent allocation policy

This commit introduces extent allocation policy for btrfs. This policy
controls how btrfs allocate an extents from block groups.  There is no
functional change introduced with this commit.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>