vmap: add flag to allow lazy unmap to be disabled at runtime

Add a flag to force lazy_max_pages() to zero to prevent any outstanding
mapped pages.  We'll need this for Xen.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Nick Piggin <npiggin@suse.de>