net: ipv6: Fix IPv6 netmask parsing

It should be possible to specify a netmask when
setting a static IPv6 address.  For example:
setenv ip6addr 2001:cafe:cafe:cafe::100/64

The net_prefix_length and net_ip6 should be updated
properly.

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/net/net6.c b/net/net6.c
index fdea078..75577bc 100644
--- a/net/net6.c
+++ b/net/net6.c
@@ -47,10 +47,13 @@
 	}
 
 	mask = strchr(value, '/');
-	len = strlen(value);
 
-	if (mask)
-		net_prefix_length = simple_strtoul(value + len, NULL, 10);
+	if (mask) {
+		net_prefix_length = simple_strtoul(mask + 1, NULL, 10);
+		len = mask - value;
+	} else {
+		len = strlen(value);
+	}
 
 	return string_to_ip6(value, len, &net_ip6);
 }