The WARN_ON() in start_transaction() was triggered while balancing.

The cause is btrfs_relocate_chunk() started a transaction and
then called iput() on the inode that stores free space cache,
and iput() called btrfs_start_transaction() again.
Reported-by: NTsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Reviewed-by: NJosef Bacik <josef@redhat.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>