btrfs: add forward declarations and headers, part 3

btrfs: add forward declarations and headers, part 3

Do a cleanup in the rest of the headers:

- add forward declarations for types referenced by pointers
- add includes when types need them

This fixes potential compilation problems if the headers are reordered
or the missing includes are not provided indirectly.

Signed-off-by: David Sterba <dsterba@suse.com>