zebra: mpls_ftn_uninstall handle nhg hash label change

zebra: mpls_ftn_uninstall handle nhg hash label change

We were crashing due to a missed label change code path
in mpls_ftn_uninstall() with the zebra_nhg hashing code.

Add a static handler function for label changing everywhere
in that code and use it in mpls_ftn_uninstall().

The crash was found in the ISIS-SR tests:

==23== Thread 1:
==23== Invalid read of size 4
==23==    at 0x15B20E: zebra_nhg_hash_equal (zebra_nhg.c:365)
==23==    by 0x489A2FD: hash_get (hash.c:143)
==23==    by 0x489A4BC: hash_lookup (hash.c:183)
==23==    by 0x15B5A3: zebra_nhg_find (zebra_nhg.c:494)
==23==    by 0x15C536: zebra_nhg_rib_find (zebra_nhg.c:1070)
==23==    by 0x1573E8: mpls_ftn_update (zebra_mpls.c:2661)
==23==    by 0x1A2554: zread_mpls_labels_replace (zapi_msg.c:1890)
==23==    by 0x1A41CD: zserv_handle_commands (zapi_msg.c:2613)
==23==    by 0x199B17: zserv_process_messages (zserv.c:517)
==23==    by 0x48EE6B7: thread_call (thread.c:1549)
==23==    by 0x48A8AD5: frr_run (libfrr.c:1064)
==23==    by 0x1391B7: main (main.c:468)
==23==  Address 0x5839330 is 0 bytes inside a block of size 80 free'd
==23==    at 0x48369AB: free (vg_replace_malloc.c:530)
==23==    by 0x48AEE6C: qfree (memory.c:129)
==23==    by 0x15C5F8: zebra_nhg_free (zebra_nhg.c:1095)
==23==    by 0x15BC8C: zebra_nhg_handle_uninstall (zebra_nhg.c:734)
==23==    by 0x15DCFA: zebra_nhg_uninstall_kernel (zebra_nhg.c:1826)
==23==    by 0x15C666: zebra_nhg_decrement_ref (zebra_nhg.c:1106)
==23==    by 0x15D9D7: zebra_nhg_re_update_ref (zebra_nhg.c:1711)
==23==    by 0x15D8B1: nexthop_active_update (zebra_nhg.c:1660)
==23==    by 0x167072: rib_process (zebra_rib.c:1154)
==23==    by 0x168D72: process_subq_route (zebra_rib.c:2039)
==23==    by 0x168E92: process_subq (zebra_rib.c:2078)
==23==    by 0x168F5B: meta_queue_process (zebra_rib.c:2112)
==23==  Block was alloc'd at
==23==    at 0x4837B65: calloc (vg_replace_malloc.c:752)
==23==    by 0x48AED56: qcalloc (memory.c:110)
==23==    by 0x15B07B: zebra_nhg_copy (zebra_nhg.c:307)
==23==    by 0x15B13E: zebra_nhg_hash_alloc (zebra_nhg.c:329)
==23==    by 0x489A339: hash_get (hash.c:148)
==23==    by 0x15B6CA: zebra_nhg_find (zebra_nhg.c:532)
==23==    by 0x15C536: zebra_nhg_rib_find (zebra_nhg.c:1070)
==23==    by 0x15D89A: nexthop_active_update (zebra_nhg.c:1658)
==23==    by 0x167072: rib_process (zebra_rib.c:1154)
==23==    by 0x168D72: process_subq_route (zebra_rib.c:2039)
==23==    by 0x168E92: process_subq (zebra_rib.c:2078)
==23==    by 0x168F5B: meta_queue_process (zebra_rib.c:2112)

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>