Revert commit 722f0b89 ("pNFS: Don't send COMMITs to the DSes if...")

Doing the test without taking any locks is racy, and so really it makes
more sense to do it in the flexfiles code (which is the only case that
cares).

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>