nfs4.1: prevent race that allowed use of freed layout in _pnfs_return_layout

mark_matching_lsegs_invalid could put the last ref to the layout, so
the get_layout_hdr needs to be called first.
Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pnfs.c

@@ -640,10 +640,10 @@ _pnfs_return_layout(struct inode *ino)
 		return status;
 	}
 	stateid = nfsi->layout->plh_stateid;
-	mark_matching_lsegs_invalid(lo, &tmp_list, NULL);
-	lo->plh_block_lgets++;
 	/* Reference matched in nfs4_layoutreturn_release */
 	get_layout_hdr(lo);
+	mark_matching_lsegs_invalid(lo, &tmp_list, NULL);
+	lo->plh_block_lgets++;
 	spin_unlock(&ino->i_lock);
 	pnfs_free_lseg_list(&tmp_list);