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>

fs/btrfs/space-info.c

@@ -232,6 +232,7 @@ static int create_space_info(struct btrfs_fs_info *info, u64 flags)
 	if (!space_info)
 		return -ENOMEM;
 
+	space_info->fs_info = info;
 	for (i = 0; i < BTRFS_NR_RAID_TYPES; i++)
 		INIT_LIST_HEAD(&space_info->block_groups[i]);
 	init_rwsem(&space_info->groups_sem);

fs/btrfs/space-info.h

@@ -94,6 +94,7 @@ enum btrfs_flush_state {
 };
 
 struct btrfs_space_info {
+	struct btrfs_fs_info *fs_info;
 	spinlock_t lock;
 
 	u64 total_bytes;	/* total bytes in the space,