btrfs: merge unlocking to common exit block in btrfs_commit_transaction

The tree_log_mutex and reloc_mutex locks are properly nested so we can
simplify error handling and add labels for them. This reduces line count
of the function.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>