opt_inet6.h was missing from some files in the new NFS subsystem.

The effect of this was, for clients mounted via inet6 addresses,
that the DRC cache would never have a hit in the server. It also
broke NFSv4 callbacks when an inet6 address was the only one available
in the client. This patch fixes the above, plus deletes opt_inet6.h
from a couple of files it is not needed for.

MFC after:	2 weeks

sys/fs/nfs/nfs_commonkrpc.c

@@ -38,7 +38,6 @@ __FBSDID("$FreeBSD$");
  * Socket operations for use by nfs
  */
 
-#include "opt_inet6.h"
 #include "opt_kdtrace.h"
 #include "opt_kgssapi.h"
 #include "opt_nfs.h"

sys/fs/nfs/nfs_commonsubs.c

@@ -40,6 +40,8 @@ __FBSDID("$FreeBSD$");
  * copy data between mbuf chains and uio lists.
  */
 #ifndef APPLEKEXT
+#include "opt_inet6.h"
+
 #include <fs/nfs/nfsport.h>
 
 /*

sys/fs/nfsclient/nfs_clkrpc.c

@@ -34,7 +34,6 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include "opt_inet6.h"
 #include "opt_kgssapi.h"
 
 #include <fs/nfs/nfsport.h>

sys/fs/nfsclient/nfs_clport.c

@@ -34,6 +34,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include "opt_inet6.h"
 #include "opt_kdtrace.h"
 
 #include <sys/capability.h>

sys/fs/nfsclient/nfs_clrpcops.c

@@ -43,6 +43,8 @@ __FBSDID("$FreeBSD$");
  */
 
 #ifndef APPLEKEXT
+#include "opt_inet6.h"
+
 #include <fs/nfs/nfsport.h>
 
 /*