btrfs: move extent-tree function declarations out of ctree.h

We have 3 functions that have their prototypes declared in ctree.h but
they are defined at extent-tree.c and they are unrelated to the btree
data structure. Move the prototypes out of ctree.h and into extent-tree.h.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>