Honor NFSv3 commit call (RFC 1813, Section 3.3.21) where when count is 0,

the full length from offset is being flushed.  Note that for now VOP_FSYNC
does not support offset and length parameters so we still do the same full
VOP_FSYNC.  This issue was reported at FreeNAS support site as FreeNAS
ticket #1096.

Submitted by:	"ceckerle" <ce.freenas eckerle net>
Prodded by:	gcooper
Reviewed by:	rmacklem
MFC after:	2 weeks

sys/nfsserver/nfs_serv.c

@@ -3454,7 +3454,12 @@ nfsrv_commit(struct nfsrv_descript *nfsd, struct nfssvc_sock *slp,
 	}
 	for_ret = VOP_GETATTR(vp, &bfor, cred);
 
-	if (cnt > MAX_COMMIT_COUNT) {
+	/*
+	 * RFC 1813 3.3.21: if count is 0, a flush from offset to the end of file
+	 * is done.  At this time VOP_FSYNC does not accept offset and byte count
+	 * parameters so call VOP_FSYNC the whole file for now.
+	 */
+	if (cnt == 0 || cnt > MAX_COMMIT_COUNT) {
 		/*
 		 * Give up and do the whole thing
 		 */