x86/ioport: add ksys_ioperm() helper; remove in-kernel calls to sys_ioperm()

Using this helper allows us to avoid the in-kernel calls to the
sys_ioperm() syscall. The ksys_ prefix denotes that this function is meant
as a drop-in replacement for the syscall. In particular, it uses the same
calling convention as sys_ioperm().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: x86@kernel.org
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/x86/kernel/ioport.c

@@ -23,7 +23,7 @@
 /*
  * this changes the io permissions bitmap in the current task.
  */
-SYSCALL_DEFINE3(ioperm, unsigned long, from, unsigned long, num, int, turn_on)
+long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
 {
 	struct thread_struct *t = &current->thread;
 	struct tss_struct *tss;
@@ -96,6 +96,11 @@ SYSCALL_DEFINE3(ioperm, unsigned long, from, unsigned long, num, int, turn_on)
 	return 0;
 }
 
+SYSCALL_DEFINE3(ioperm, unsigned long, from, unsigned long, num, int, turn_on)
+{
+	return ksys_ioperm(from, num, turn_on);
+}
+
 /*
  * sys_iopl has to be used when you want to access the IO ports
  * beyond the 0x3ff range: to get the full 65536 ports bitmapped