btrfs: move a few exported extent_io_tree helpers to extent-io-tree.c

These are the last few helpers that do not rely on tree_search() and
who's other helpers are exported and in extent-io-tree.c already.  Move
these across now in order to make the core move smaller.

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