NFSv4/flexfiles: Fix invalid deref in FF_LAYOUT_DEVID_NODE()

If the attempt to instantiate the mirror's layout DS pointer failed,
then that pointer may hold a value of type ERR_PTR(), so we need
to check that before we dereference it.

Fixes: 65990d1a ("pNFS/flexfiles: Fix a deadlock on LAYOUTGET")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/flexfilelayout/flexfilelayout.h

@@ -132,16 +132,6 @@ FF_LAYOUT_LSEG(struct pnfs_layout_segment *lseg)
 			    generic_hdr);
 }
 
-static inline struct nfs4_deviceid_node *
-FF_LAYOUT_DEVID_NODE(struct pnfs_layout_segment *lseg, u32 idx)
-{
-	if (idx >= FF_LAYOUT_LSEG(lseg)->mirror_array_cnt ||
-	    FF_LAYOUT_LSEG(lseg)->mirror_array[idx] == NULL ||
-	    FF_LAYOUT_LSEG(lseg)->mirror_array[idx]->mirror_ds == NULL)
-		return NULL;
-	return &FF_LAYOUT_LSEG(lseg)->mirror_array[idx]->mirror_ds->id_node;
-}
-
 static inline struct nfs4_ff_layout_ds *
 FF_LAYOUT_MIRROR_DS(struct nfs4_deviceid_node *node)
 {
@@ -151,9 +141,25 @@ FF_LAYOUT_MIRROR_DS(struct nfs4_deviceid_node *node)
 static inline struct nfs4_ff_layout_mirror *
 FF_LAYOUT_COMP(struct pnfs_layout_segment *lseg, u32 idx)
 {
-	if (idx >= FF_LAYOUT_LSEG(lseg)->mirror_array_cnt)
+	struct nfs4_ff_layout_segment *fls = FF_LAYOUT_LSEG(lseg);
+
+	if (idx < fls->mirror_array_cnt)
+		return fls->mirror_array[idx];
+	return NULL;
+}
+
+static inline struct nfs4_deviceid_node *
+FF_LAYOUT_DEVID_NODE(struct pnfs_layout_segment *lseg, u32 idx)
+{
+	struct nfs4_ff_layout_mirror *mirror = FF_LAYOUT_COMP(lseg, idx);
+
+	if (mirror != NULL) {
+		struct nfs4_ff_layout_ds *mirror_ds = mirror->mirror_ds;
+
+		if (!IS_ERR_OR_NULL(mirror_ds))
+			return &mirror_ds->id_node;
+	}
 	return NULL;
-	return FF_LAYOUT_LSEG(lseg)->mirror_array[idx];
 }
 
 static inline u32