GFS2: dirty inode correctly in gfs2_write_end

GFS2 was only setting I_DIRTY_DATASYNC on files that it wrote to, when
it actually increased the file size.  If gfs2_fsync was called without
I_DIRTY_DATASYNC set, it didn't flush the incore data to the log before
returning, so any metadata or journaled data changes were not getting
fsynced. This meant that writes to the middle of files were not always
getting fsynced properly.

This patch makes gfs2 set I_DIRTY_DATASYNC whenever metadata has been
updated during a write. It also make gfs2_sync flush the incore log
if I_DIRTY_PAGES is set, and the file is using data journalling. This
will make sure that all incore logged data gets written to disk before
returning from a fsync.

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