btrfs: mark sanity checks when getting chunk map as unlikely

When getting a chunk map, at btrfs_get_chunk_map(), we do some sanity
checks to verify that we found an extent map and that it includes the
requested logical address. These are never expected to fail, so mark
them as unlikely to make it more clear as well as to allow a compiler
to generate more efficient code.

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