itimers: Move compat itimer syscalls to native ones

get rid of set_fs(), sanitize compat copyin/copyout.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170607084241.28657-12-viro@ZenIV.linux.org.uk

diff --git a/include/linux/compat.h b/include/linux/compat.h
index ecb8dd2..425563c 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -94,6 +94,10 @@ struct compat_itimerval {
 	struct compat_timeval	it_value;
 };
 
+struct itimerval;
+int get_compat_itimerval(struct itimerval *, const struct compat_itimerval __user *);
+int put_compat_itimerval(struct compat_itimerval __user *, const struct itimerval *);
+
 struct compat_tms {
 	compat_clock_t		tms_utime;
 	compat_clock_t		tms_stime;