lib: Report IPv6 MTU and not IPv4 for if_update_state_mtu6

[mwinter-osr]

Original PR21501 by ton31337

[greptile-apps]

Greptile Summary

This is a one-line bug fix in lib/if.c correcting a copy-paste error in if_update_state_mtu6: the function was assigning the incoming mtu parameter to ifp->mtu6 correctly, but then passing ifp->mtu (the IPv4 MTU) instead of ifp->mtu6 (the IPv6 MTU) to nb_op_updatef for the operational-state notification. The fix is correct and mirrors the pattern used in the companion if_update_state_mtu function.

Confidence Score: 5/5

• Safe to merge — trivial one-line copy-paste bug fix with no side effects.
• The change is a single-variable correction (ifp->mtu → ifp->mtu6) that is obviously correct by inspection and consistent with the parallel if_update_state_mtu function. Existing mgmt_oper topotests cover the mtu6 operational-state field. No CLI changes, no user-visible behavioral change beyond reporting the correct IPv6 MTU value. No P0 or P1 findings.
• No files require special attention.

Important Files Changed

Filename	Overview
lib/if.c	One-line bug fix: nb_op_updatef in if_update_state_mtu6 was passing ifp->mtu (IPv4) instead of ifp->mtu6 (IPv6) for the mtu6 operational-state notification. Fix is correct and consistent with if_update_state_mtu.

Sequence Diagram

sequenceDiagram
    participant Zebra as zebra/interface.c
    participant IfLib as lib/if.c
    participant NB as nb_op_updatef (MGMT)

    Zebra->>IfLib: if_update_state_mtu6(ifp, mtu)
    IfLib->>IfLib: "ifp->mtu6 = mtu"
    Note over IfLib: BEFORE: passed ifp->mtu (IPv4!)
    Note over IfLib: AFTER: passes ifp->mtu6 (IPv6 ✓)
    IfLib->>NB: "nb_op_updatef(state, "mtu6", "%u", ifp->mtu6)"
    NB-->>Zebra: operational state updated with correct IPv6 MTU

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