fs: FileBlockManager should sync clean blocks too
fs: FileBlockManager should sync clean blocks too

Otherwise empty blocks may not be made durable. And we want them to be.

Change-Id: I9d0838a20767376846a536b01545e6b6fb54b0cc
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4991
Tested-by: jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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