fs: Add gflag to control data block fsync, enable by default
fs: Add gflag to control data block fsync, enable by default

Change-Id: I5a6636d8cf038d449144ac4cc4fc30026e8a2c82
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4232
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: jenkins

InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu