net/9p: Implement attrwalk 9p call

TXATTRWALK: Descend a ATTR namespace

 size[4] TXATTRWALK tag[2] fid[4] newfid[4] name[s]
 size[4] RXATTRWALK tag[2] size[8]

txattrwalk gets a fid pointing to xattr. This fid can later be
used to read the xattr value. If name is NULL the fid returned
can be used to get the list of extended attribute associated to
the file system object.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -1622,6 +1622,56 @@ error:
 }
 EXPORT_SYMBOL(p9_client_rename);
 
+/*
+ * An xattrwalk without @attr_name gives the fid for the lisxattr namespace
+ */
+struct p9_fid *p9_client_xattrwalk(struct p9_fid *file_fid,
+				const char *attr_name, u64 *attr_size)
+{
+	int err;
+	struct p9_req_t *req;
+	struct p9_client *clnt;
+	struct p9_fid *attr_fid;
+
+	err = 0;
+	clnt = file_fid->clnt;
+	attr_fid = p9_fid_create(clnt);
+	if (IS_ERR(attr_fid)) {
+		err = PTR_ERR(attr_fid);
+		attr_fid = NULL;
+		goto error;
+	}
+	P9_DPRINTK(P9_DEBUG_9P,
+		">>> TXATTRWALK file_fid %d, attr_fid %d name %s\n",
+		file_fid->fid, attr_fid->fid, attr_name);
+
+	req = p9_client_rpc(clnt, P9_TXATTRWALK, "dds",
+			file_fid->fid, attr_fid->fid, attr_name);
+	if (IS_ERR(req)) {
+		err = PTR_ERR(req);
+		goto error;
+	}
+	err = p9pdu_readf(req->rc, clnt->proto_version, "q", attr_size);
+	if (err) {
+		p9pdu_dump(1, req->rc);
+		p9_free_req(clnt, req);
+		goto clunk_fid;
+	}
+	p9_free_req(clnt, req);
+	P9_DPRINTK(P9_DEBUG_9P, "<<<  RXATTRWALK fid %d size %llu\n",
+		attr_fid->fid, *attr_size);
+	return attr_fid;
+clunk_fid:
+	p9_client_clunk(attr_fid);
+	attr_fid = NULL;
+error:
+	if (attr_fid && (attr_fid != file_fid))
+		p9_fid_destroy(attr_fid);
+
+	return ERR_PTR(err);
+}
+EXPORT_SYMBOL_GPL(p9_client_xattrwalk);
+
 int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset)
 {
 	int err, rsize, total;