address_space_write: address_space_to_flatview needs RCU lock
    
    address_space_write 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_write to address_space_write.
    
    Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    (cherry picked from commit 4c6ebbb364aa6f42c5d8e83e932e967eb83f0e44)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>