decnet: Fix complier warning

[Steven Whitehouse <swhiteho@redhat.com>]

This patch fixes a warning seen during compiling dn_dev.c. The warning is harmless
since we know that the structure length in question is less than 2^32 bytes, but
this fixes it anyway.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Christine Caulfield <ccaulfie@redhat.com>

diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index 2f0ac3c..d9bc69c 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -152,7 +152,7 @@ static struct dn_dev_parms dn_dev_list[] =  {
 
 #define DN_DEV_LIST_SIZE ARRAY_SIZE(dn_dev_list)
 
-#define DN_DEV_PARMS_OFFSET(x) ((int) ((char *) &((struct dn_dev_parms *)0)->x))
+#define DN_DEV_PARMS_OFFSET(x) ((long) ((char *) &((struct dn_dev_parms *)0)->x))
 
 #ifdef CONFIG_SYSCTL