GFS2: Use filemap_fdatawrite() to write back the AIL

In order to ensure that the mapping stats (and thus the bdi) are correctly
updated, this patch changes the AIL writeback to use the filemap_datawrite
function. This helps prevent stalls in balance_dirty_pages() due to
large amounts of dirty metadata when there is little or no dirty data
around.

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