btrfs: move the fs_info related helpers closer to fs_info in ctree.h

This is purely cosmetic, to make it straightforward to copy and paste
the definition and helpers from ctree.h into fs.h.  These are helpers
that act directly on the fs_info, and were scattered throughout ctree.h.
Move them directly below the fs_info definition to make it easier to
move them later.  This includes the exclop prototypes, which shares an
enum that's used in struct btrfs_fs_info as well.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>