zebra: Optimize invoking nhg compare func

zebra: Optimize invoking nhg compare func

In some cases, the old_re nhe and the newnhe is same and there is no
point in comparing them both since they are the same. Skip comparing in
such cases.

Ex:
2025/01/09 23:49:27.489020 ZEBRA: [W4Z4R-NTSMD] zebra_nhg_rib_find_nhe: => nhe 0x555f611d30c0 (44[38/39/45])
2025/01/09 23:49:27.489021 ZEBRA: [ZH3FQ-TE9NV] zebra_nhg_rib_compare_old_nhe: 0.0.0.0/0 new id: 44 old id: 44
2025/01/09 23:49:27.489021 ZEBRA: [YB8HE-Z86GN] zebra_nhg_rib_compare_old_nhe: 0.0.0.0/0 NEW 0x555f611d30c0 (44[38/39/45])
2025/01/09 23:49:27.489023 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 20.1.1.9[0] vrf default(0) wgt 1, with flags
2025/01/09 23:49:27.489024 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 30.1.2.9[0] vrf default(0) wgt 1, with flags
2025/01/09 23:49:27.489025 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 20.1.1.2[4] vrf default(0) wgt 1, with flags ACTIVE
2025/01/09 23:49:27.489026 ZEBRA: [ZM3BX-HPETZ] zebra_nhg_rib_compare_old_nhe: 0.0.0.0/0 OLD 0x555f611d30c0 (44[38/39/45])
2025/01/09 23:49:27.489027 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 20.1.1.9[0] vrf default(0) wgt 1, with flags
2025/01/09 23:49:27.489028 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 30.1.2.9[0] vrf default(0) wgt 1, with flags
2025/01/09 23:49:27.489028 ZEBRA: [ZSB1Z-XM2V3] 0.0.0.0/0: NH 20.1.1.2[4] vrf default(0) wgt 1, with flags ACTIVE

Signed-off-by: Rajasekar Raja <rajasekarr@nvidia.com>