gfs2: less aggressive low-memory log flushing

gfs2: less aggressive low-memory log flushing

It turns out that for some workloads, the fix in commit b74cd55aa9a9d
("gfs2: low-memory forced flush fixes") causes the number of forced log
flushes to increase to a degree that the overall filesystem performance
drops significantly.  Address that by forcing a log flush only when
gfs2_writepages cannot make any progress rather than when it cannot make
"enough" progress.

Fixes: b74cd55aa9a9d ("gfs2: low-memory forced flush fixes")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>