[PATCH] xfs: dummy transactions should not dirty VFS state

[Christoph Hellwig]

I looks good to me, but there are two subtile differences between
xfs_commit_dummy_trans and xfs_fs_log_dummy that get lost.  For one
xfs_commit_dummy_trans doesn't actually commit a synchronous transaction
(or rather forces out the log) unless SYNC_WAIT is set, in addition
to that xfs_fs_log_dummy uses _xfs_trans_alloc, which doesn't get
blocked by the filesystem freezing.