tipc: silence sparse warnings

Eliminate below sparse warnings:

net/tipc/link.c:1210:37: warning: cast removes address space of expression
net/tipc/link.c:1218:59: warning: incorrect type in argument 2 (different address spaces)
net/tipc/link.c:1218:59:    expected void const [noderef] <asn:1>*from
net/tipc/link.c:1218:59:    got unsigned char const [usertype] *[assigned] sect_crs
net/tipc/socket.c:341:49: warning: Using plain integer as NULL pointer
net/tipc/socket.c:1371:36: warning: Using plain integer as NULL pointer
net/tipc/socket.c:1694:57: warning: Using plain integer as NULL pointer

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Andreas Bofjäll <andreas.bofjall@ericsson.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/link.c b/net/tipc/link.c
index b02a6dc..be73a1f 100644
--- a/net/tipc/link.c
+++ b/net/tipc/link.c
@@ -1160,7 +1160,7 @@
 	struct tipc_msg fragm_hdr;
 	struct sk_buff *buf, *buf_chain, *prev;
 	u32 fragm_crs, fragm_rest, hsz, sect_rest;
-	const unchar *sect_crs;
+	const unchar __user *sect_crs;
 	int curr_sect;
 	u32 fragm_no;
 	int res = 0;
@@ -1202,7 +1202,7 @@
 
 		if (!sect_rest) {
 			sect_rest = msg_sect[++curr_sect].iov_len;
-			sect_crs = (const unchar *)msg_sect[curr_sect].iov_base;
+			sect_crs = msg_sect[curr_sect].iov_base;
 		}
 
 		if (sect_rest < fragm_rest)