btrfs: fix chunk map leak in btrfs_map_block() after btrfs_chunk_map_num_copies()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix chunk map leak in btrfs_map_block() after btrfs_chunk_map_num_copies()

Fix a chunk map leak in btrfs_map_block(): if we return early with -EINVAL,

we're not freeing the chunk map that we've just looked up.