Ignore sync requests from the system syncher, i.e. VFS_SYNC(waitfor=MNT_LAZY).

ZFS already commits outstanding data every zfs_txg_timeout seconds, so these
syncs are unnecessarily intrusive.

Submitted by:	gibbs
Sponsored by:	Spectra Logic
MFSpectraBSD:	1105759 on 2014/12/11

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c

@@ -132,6 +132,13 @@ zfs_sync(vfs_t *vfsp, int waitfor)
 	if (panicstr)
 		return (0);
 
+	/*
+	 * Ignore the system syncher.  ZFS already commits async data
+	 * at zfs_txg_timeout intervals.
+	 */
+	if (waitfor == MNT_LAZY)
+		return (0);
+
 	if (vfsp != NULL) {
 		/*
 		 * Sync a specific filesystem.