exec.c: get nodes_nb_alloc with one MAX calculation

The purpose of these two MAX here is to get the maximum of these three
variables:

    A: map->nodes_nb + nodes
    B: map->nodes_nb_alloc
    C: alloc_hint

We can write it like MAX(A, B, C). Since the if condition says A > B,
this means MAX(A, B, C) = MAX(A, C).

This patch just simplify the calculation a bit.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190321082555.21118-4-richardw.yang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>