GFS2: Split data write & wait in fsync

Now that the data writing is part of fsync proper, we can split
the waiting part out and do it later on. This reduces the
number of waits that we do during fsync on average.

There is also no need to take the i_mutex unless we are flushing
metadata to disk, so we can move that to within the metadata
flushing code.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>