x86: implement mapping_error in pci-nommu_64.c

This patch implements mapping_error for pci-nommu_64.c.
It takes care to keep the same compatible behaviour it already
had. Although this file is not (yet) used for i386, we introduce
the i386 version here. Again, care is taken, even at the expense of
an ifdef, to keep the same behaviour inconditionally.

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