btrfs: avoid create and commit empty transaction when committing super

At btrfs_commit_super(), called in a few contexts such as when unmounting
a filesystem, we use btrfs_join_transaction() to catch any running
transaction and then commit it. This will however create a new and empty
transaction in case there's no running transaction or there's a running
transaction with a state >= TRANS_STATE_UNBLOCKED.

As we just want to be sure that any existing transaction is fully
committed, we can use btrfs_attach_transaction_barrier() instead of
btrfs_join_transaction(), therefore avoiding the creation and commit of
empty transactions, which only waste IO and causes rotation of the
precious backup roots.

Example where we create and commit a pointless empty transaction:

  $ mkfs.btrfs -f /dev/sdj
  $ btrfs inspect-internal dump-super /dev/sdj | grep -e '^generation'
  generation            6

  $ mount /dev/sdj /mnt/sdj
  $ touch /mnt/sdj/foo

  # Commit the currently open transaction. Just 'sync' or wait ~30
  # seconds for the transaction kthread to commit it.
  $ sync

  $ btrfs inspect-internal dump-super /dev/sdj | grep -e '^generation'
  generation            7

  $ umount /mnt/sdj

  $ btrfs inspect-internal dump-super /dev/sdj | grep -e '^generation'
  generation            8

The transaction with id 8 was pointless, an empty transaction that did
not achieve anything.

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