cifs: Include linux/err.h for IS_ERR and PTR_ERR

Fixes build errors:

net/dns_resolver/dns_key.c: In function 'init_dns_resolver':
net/dns_resolver/dns_key.c:170: error: implicit declaration of function 'IS_ERR'
net/dns_resolver/dns_key.c:171: error: implicit declaration of function 'PTR_ERR'
net/dns_resolver/dns_query.c: In function 'dns_query':
net/dns_resolver/dns_query.c:126: error: implicit declaration of function 'IS_ERR'
net/dns_resolver/dns_query.c:127: error: implicit declaration of function 'PTR_ERR'
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

net/dns_resolver/dns_key.c

@@ -28,6 +28,7 @@
 #include <linux/string.h>
 #include <linux/kernel.h>
 #include <linux/keyctl.h>
+#include <linux/err.h>
 #include <keys/dns_resolver-type.h>
 #include <keys/user-type.h>
 #include "internal.h"

net/dns_resolver/dns_query.c

@@ -39,6 +39,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/dns_resolver.h>
+#include <linux/err.h>
 #include <keys/dns_resolver-type.h>
 #include <keys/user-type.h>