mds: disallow dumping huge caches to formatter

mds: disallow dumping huge caches to formatter

Dumping huge caches (~ 1G) to formatter causes mds to
hang or get terminated. Until the underlying issues is
fixed, disallow dumping cache if cache usage exceeds this
threshold. Also, Patrick feels a similar issue might be
hiding there for cache dumps to file when cache sizes
are really huge. This case has not been been reproduced
yet, hence, the limit for cache dumps to file is currently
kept not capped.

Fixes: http://tracker.ceph.com/issues/36703
Signed-off-by: Venky Shankar <vshankar@redhat.com>