pNFS: Handle allocation errors correctly in filelayout_alloc_layout_hdr()

Return the NULL pointer when the allocation fails.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: <stable@vger.kernel.org> # 3.5.x
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4filelayout.c

@@ -1348,7 +1348,7 @@ filelayout_alloc_layout_hdr(struct inode *inode, gfp_t gfp_flags)
 	struct nfs4_filelayout *flo;
 
 	flo = kzalloc(sizeof(*flo), gfp_flags);
-	return &flo->generic_hdr;
+	return flo != NULL ? &flo->generic_hdr : NULL;
 }
 
 static void