HACK: suites/fs: set tiny cache sizes

Should exercise cache limits/trimming without having
to create workloads with crazy numbers of files.

This is an exploratory test, we should come up with
something more nuanced that doesn't apply to all tests, as this
will make at least some of them really slow, probably.

Signed-off-by: John Spray <john.spray@redhat.com>

suites/fs/basic/dirfrag/frag_enable.yaml

@@ -2,10 +2,14 @@
 overrides:
   ceph:
     conf:
+      client:
+        client cache size: 100
       mds:
         mds bal frag: true
-        mds bal fragment size max: 10000
-        mds bal split size: 100
+        mds bal fragment size max: 50
+        mds bal split size: 20
         mds bal merge size: 5
         mds bal split bits: 3
 
+        mds cache size: 1000
+