btrfs: move btrfs_debug_check_extent_io_range into extent-io-tree.c

This helper is used by a lot of the core extent_io_tree helpers, so
temporarily export it and move it into extent-io-tree.c in order to make
it straightforward to migrate the helpers in batches.

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