btrfs: simplify mutex unlocking code in btrfs_commit_transaction

No functional change rearrange the mutex_unlock.

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