NFSv4.1: put_layout_hdr can remove nfsi->layout

Prevents an Oops triggered by CB_LAYOUTRECALL and LAYOUTGET race on a
pnfs_layout_hdr first pnfs_layout_segment.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pnfs.c

@@ -768,7 +768,7 @@ pnfs_update_layout(struct inode *ino,
 	put_layout_hdr(lo);
 out:
 	dprintk("%s end, state 0x%lx lseg %p\n", __func__,
-		nfsi->layout->plh_flags, lseg);
+		nfsi->layout ? nfsi->layout->plh_flags : -1, lseg);
 	return lseg;
 out_unlock:
 	spin_unlock(&ino->i_lock);