Remove cpumask for page_info struct.

Remove cpumask for page_info struct.

This makes TLB flushing on page allocation more conservative, but the
flush clock should still save us most of the time (page freeing and
alloc'ing tends to happen in batches, and not necesasrily close
together). We could add some optimisations to the flush filter if this
does turn out to be a significant overhead for some (useful)
workloads.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>