address_space_read: address_space_to_flatview needs RCU lock

address_space_read: address_space_to_flatview needs RCU lock

address_space_read is calling address_space_to_flatview but it can
be called outside the RCU lock.  To fix it, push the rcu_read_lock/unlock
pair up from flatview_read_full to address_space_read's constant size
fast path and address_space_read_full.

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit b2a44fcad74f1cc7a6786d38eba7db12ab2352ba)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>