xen/mmu: inhibit vmap aliases rather than trying to clear them out

Rather than trying to deal with aliases once they appear, just completely
inhibit them.  Mostly the removal of aliases was managable, but it comes
unstuck in xen_create_contiguous_region() because it gets executed at
interrupt time (as a result of dma_alloc_coherent()), which causes all
sorts of confusion in the vmap code, as it was never intended to be run
in interrupt context.

This has the unfortunate side effect of removing all the unmap batching
the vmap code so carefully added, but that can't be helped.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>