bgpd:add correct value into dfAlgorithm json output

bgpd:add correct value into dfAlgorithm json output

FRR "show bgp l2 evpn es <esi-id> json" output shows 'dfAlgorithm'
same as 'dfPreference' value instead of algorithm.

Modified the code to set the correct DF algorithm value into
'dfAlgorithm' field.

torm-11# show bgp l2 evpn es
ES Flags: B - bypass, L local, R remote, I inconsistent
VTEP Flags: E ESR/Type-4, A active nexthop
ESI                            Flags RD                    #VNIs
VTEPs
03:44:38:39:ff:ff:01:00:00:01  LR    27.0.0.15:15          10
27.0.0.16(EA),27.0.0.17(EA)
03:44:38:39:ff:ff:01:00:00:02  LR    27.0.0.15:16          10
27.0.0.16(EA),27.0.0.17(EA)
03:44:38:39:ff:ff:01:00:00:03  LR    27.0.0.15:17          10
27.0.0.16(EA),27.0.0.17(EA)
03:44:38:39:ff:ff:02:00:00:01  R     -                     10
27.0.0.18(A),27.0.0.19(A),27.0.0.20(A)
03:44:38:39:ff:ff:02:00:00:02  R     -                     10
27.0.0.18(A),27.0.0.19(A),27.0.0.20(A)
03:44:38:39:ff:ff:02:00:00:03  R     -                     10
27.0.0.18(A),27.0.0.19(A),27.0.0.20(A)
torm-11#

Before Fix:-
```
torm-11# show bgp l2 evpn es 03:44:38:39:ff:ff:01:00:00:01 json
{
  "esi":"03:44:38:39:ff:ff:01:00:00:01",
  "rd":"27.0.0.15:15",
  "type":[
    "local",
    "remote"
  ],
  "vteps":[
    {
      "vtep_ip":"27.0.0.16",
      "flags":[
        "esr",
        "active"
      ],
      "dfPreference":32767,
      "dfAlgorithm":32767 =====> dfAlgorithm is same as dfPreference
    },
    {
      "vtep_ip":"27.0.0.17",
      "flags":[
        "esr",
        "active"
      ],
      "dfPreference":32767,
      "dfAlgorithm":32767 =====> dfAlgorithm is same as dfPreference
    }
  ],
  "flags":[
    "up",
    "advertiseEVI"
  ],
  "originator_ip":"27.0.0.15",
  "remoteVniCount":10,
  "vrfCount":3,
  "macipPathCount":33,
  "macipGlobalPathCount":264,
  "inconsistentVniVtepCount":0,
  "localEsDfPreference":50000,
  "fragments":[
    {
      "rd":"27.0.0.15:15",
      "eviCount":10
    }
  ]
}
torm-11#
```

After Fix:-

```
torm-11# show bgp l2vpn evpn es 03:44:38:39:ff:ff:01:00:00:01 json
{
  "esi":"03:44:38:39:ff:ff:01:00:00:01",
  "rd":"27.0.0.15:4",
  "type":[
    "local",
    "remote"
  ],
  "vteps":[
    {
      "vtep_ip":"27.0.0.16",
      "flags":[
        "esr",
        "active"
      ],
      "dfPreference":32767,
      "dfAlgorithm":"preference" ---- dfAlgorithm shows properly
    },
    {
      "vtep_ip":"27.0.0.17",
      "flags":[
        "esr",
        "active"
      ],
      "dfPreference":32767,
      "dfAlgorithm":"preference" --- dfAlgorithm shows properly
    }
  ],
  "vniCount":10,
  "flags":[
    "up",
    "advertiseEVI"
  ],
  "originator_ip":"27.0.0.15",
  "remoteVniCount":10,
  "vrfCount":3,
  "macipPathCount":33,
  "macipGlobalPathCount":264,
  "inconsistentVniVtepCount":0,
  "localEsDfPreference":50000,
  "fragments":[
    {
      "rd":"27.0.0.15:4",
      "eviCount":10
    }
  ]
}
torm-11#
```

Ticket:#3411912

Issue:3411912

Testing: UT done

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>