gfs2: Don't start unnecessary transactions during log flush

Commit 8d391972 ("gfs2: Remove __gfs2_writepage()") changed the log
flush code in gfs2_ail1_start_one() to call aops->writepages() instead
of aops->writepage().  For jdata inodes, this means that we will now try
to reserve log space and start a transaction before we can determine
that the pages in question have already been journaled.  When this
happens in the context of gfs2_logd(), it can now appear that not enough
log space is available for freeing up log space, and we will lock up.

Fix that by issuing journal writes directly instead of going through
aops->writepages() in the log flush code.

Fixes: 8d391972 ("gfs2: Remove __gfs2_writepage()")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>