btrfs: fix error pointer check in __btrfs_map_block()

The btrfs_get_chunk_map() never returns NULL, it returns error pointers.

Fixes: 89b798ad ("btrfs: Use btrfs_get_io_geometry appropriately")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>