btrfs: move btrfs_fs_info declarations into fs.h

Now that we have a lot of the fs_info related helpers and stuff
isolated, copy these over to fs.h out of ctree.h.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ reformat comments ]
Signed-off-by: David Sterba <dsterba@suse.com>