gfs2: Dirty source inode during rename

Mark the source inode dirty during a rename instead of just updating the
underlying buffer head.  Otherwise, fsync may find the inode clean and
will then skip flushing the journal.  A subsequent power failure will
cause the rename to be lost.  This happens in command sequences like:

  xfs_io -f -c 'pwrite 0 4096' -c 'fsync' foo
  mv foo bar
  xfs_io -c 'fsync' bar
  # power failure

Fixes xfstests generic/322, generic/376.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>