btrfs: add struct declarations in dev-replace.h

dev-replace.h just has function prototypes for device replace, however
if you happen to include it in the wrong order you'll get compile errors
because of different structures not being defined.  Since these are just
pointer args to functions we can declare them at the top in order to
reduce the pain of using the header.

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>