x86: use WARN_ON in mapping functions

In the very same way i386 do, we use WARN_ON functions
in map_simple and map_sg.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>