To address the potential CWE-787 (Out-of-bounds Write) vulnerability in the given code slice, here are five possible patches:

Patch 1: Add a size check before memcpy
```c
20     if (nexthop_mac && sizeof(nexthop_mac) >= ETH_ALEN) {
21         memcpy(nexthop_mac, neigh->ha, ETH_ALEN);
22     } else {
23         return -EINVAL;
24     }
```

Patch 2: Use a safer memory copy function
```c
20     if (nexthop_mac) {
21         memcpy_safe(nexthop_mac, neigh->ha, ETH_ALEN);
22     } else {
23         return -EINVAL;
24     }
```

Patch 3: Use explicit array indexing instead of memcpy
```c
20     if (nexthop_mac) {
21         for (int i = 0; i < ETH_ALEN; i++) {
22             nexthop_mac[i] = neigh->ha[i];
23         }
24     } else {
25         return -EINVAL;
26     }
```

Patch 4: Add a parameter for the size of nexthop_mac
```c
1  int smc_ib_find_route(struct net *net, __be32 saddr, __be32 daddr,
2                        u8 nexthop_mac[], size_t nexthop_mac_size, u8 *uses_gateway)
3  {
   ...
20     if (nexthop_mac && nexthop_mac_size >= ETH_ALEN) {
21         memcpy(nexthop_mac, neigh->ha, ETH_ALEN);
22     } else {
23         return -EINVAL;
24     }
```

Patch 5: Use strncpy with explicit null termination
```c
20     if (nexthop_mac) {
21         strncpy((char *)nexthop_mac, (char *)neigh->ha, ETH_ALEN);
22         nexthop_mac[ETH_ALEN - 1] = '\0';
23     } else {
24         return -EINVAL;
25     }
```

Each of these patches aims to prevent potential buffer overflow by adding checks, using safer functions, or changing the approach to copying data. The choice of patch depends on the specific requirements and constraints of the system.