btrfs: temporarily export and move core extent_io_tree tree functions

A lot of the various internals of extent_io_tree call these two
functions for insert or searching the rb tree for entries, so
temporarily export them and then move them to extent-io-tree.c.  We
can't move tree_search() without renaming it, and I don't want to
introduce a bunch of churn just to do that, so move these functions
first and then we can move a few big functions and then the remaining
users of tree_search().

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