powerpc/pci: mmap anonymous memory when legacy_mem doesn't exist

The new legacy_mem file in sysfs is causing problems with X on machines
that don't support legacy memory access. The way I initially implemented
it, we would fail with -ENXIO when trying to mmap it, thus exposing to
X that we do support the API but there is no legacy memory.

Unfortunately, X poor error handling is causing it to fail to start when
it gets this error.

This implements a workaround hack that instead maps anonymous memory
instead (using shmem if VM_SHARED is set, just like /dev/zero does).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>