pnfs/flexfiles: Fix PTR_ERR() dereferences in ff_layout_track_ds_error

mirror->mirror_ds can be NULL if uninitialised, but can contain
a PTR_ERR() if call to GETDEVICEINFO failed.

Fixes: 65990d1a ("pNFS/flexfiles: Fix a deadlock on LAYOUTGET")
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # 4.10+

fs/nfs/flexfilelayout/flexfilelayoutdev.c

@@ -257,7 +257,7 @@ int ff_layout_track_ds_error(struct nfs4_flexfile_layout *flo,
 	if (status == 0)
 		return 0;
 
-	if (mirror->mirror_ds == NULL)
+	if (IS_ERR_OR_NULL(mirror->mirror_ds))
 		return -EINVAL;
 
 	dserr = kmalloc(sizeof(*dserr), gfp_flags);