NFSD: remove unneeded linux/user_namespace.h include

After commit 4c1e1b34 ("nfsd: Store ex_anon_uid and ex_anon_gid as
kuids and kgids") using kuid/kgid for ex_anon_uid/ex_anon_gid,
user_namespace.h is not needed.
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/auth.c

 /* Copyright (C) 1995, 1996 Olaf Kirch <okir@monad.swb.de> */
 
 #include <linux/sched.h>
-#include <linux/user_namespace.h>
 #include "nfsd.h"
 #include "auth.h"