btrfs: store fs_info in space_info

This is handy when computing space_info dynamic reclaim thresholds where
we do not have access to a block group. We could add it to the various
functions as a parameter, but it seems reasonable for space_info to have
an fs_info pointer.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>