btrfs: unexport btrfs_debug_check_extent_io_range

We no longer need to export this as all users are in extent-io-tree.c,
remove it from the header and put it into extent-io-tree.c.

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