zebra: Fix neigh delete causing heap-use-after-free error

zebra: Fix neigh delete causing heap-use-after-free error

Issue:
Not freeing the neighbor n  within the same function can lead to
memory leak.
zebra_neigh_del_all() -> zebra_neigh_del() re lookup and free

Fix: not accessing n after its freed.
Directly free the neighbor entry (n) when its interface index matches
ifp->ifindex.

This fixes:
ERROR: AddressSanitizer: heap-use-after-free on address 0x6070001052e8 at pc 0x7f6bf7d09ddb bp 0x7ffd3366a000 sp 0x7ffd33669ff0
READ of size 8 at 0x6070001052e8 thread T0
    #0 0x7f6bf7d09dda in _rb_next lib/openbsd-tree.c:455
    #1 0x55f95a307261 in zebra_neigh_rb_head_RB_NEXT zebra/zebra_neigh.h:34
    #2 0x55f95a3082e9 in zebra_neigh_del_all zebra/zebra_neigh.c:162
    #3 0x55f95a121ee7 in zebra_interface_down_update zebra/redistribute.c:571
    #4 0x55f95a0f819d in if_down zebra/interface.c:1017
    #5 0x55f95a0fe168 in zebra_if_dplane_ifp_handling zebra/interface.c:2102
    #6 0x55f95a0ff10c in zebra_if_dplane_result zebra/interface.c:2241
    #7 0x55f95a27ce9c in rib_process_dplane_results zebra/zebra_rib.c:5015
    #8 0x7f6bf7da3ad9 in event_call lib/event.c:1984
    #9 0x7f6bf7c62141 in frr_run lib/libfrr.c:1246
    #10 0x55f95a11ca7f in main zebra/main.c:543
    #11 0x7f6bf7029d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #12 0x7f6bf7029e3f in __libc_start_main_impl ../csu/libc-start.c:392
    #13 0x55f95a0dd0b4 in _start (/usr/lib/frr/zebra+0x1a80b4)

Ticket: #18047

Signed-off-by: Rajesh Varatharaj <rvaratharaj@nvidia.com>