zebra: Fix label manager memory leak

zebra: Fix label manager memory leak

==25402==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x533302 in calloc (/usr/lib/frr/zebra+0x533302)
    #1 0x7fee84cdc80b in qcalloc /home/qlyoung/frr/lib/memory.c:110:27
    #2 0x5a3032 in create_label_chunk /home/qlyoung/frr/zebra/label_manager.c:188:3
    #3 0x5a3c2b in assign_label_chunk /home/qlyoung/frr/zebra/label_manager.c:354:8
    #4 0x5a2a38 in label_manager_get_chunk /home/qlyoung/frr/zebra/label_manager.c:424:9
    #5 0x5a1412 in hook_call_lm_get_chunk /home/qlyoung/frr/zebra/label_manager.c:60:1
    #6 0x5a1412 in lm_get_chunk_call /home/qlyoung/frr/zebra/label_manager.c:81:2
    #7 0x72a234 in zread_get_label_chunk /home/qlyoung/frr/zebra/zapi_msg.c:2026:2
    #8 0x72a234 in zread_label_manager_request /home/qlyoung/frr/zebra/zapi_msg.c:2073:4
    #9 0x73150c in zserv_handle_commands /home/qlyoung/frr/zebra/zapi_msg.c:2688:2

When creating label chunk that has a specified base, we eventually are
calling assign_specific_label_chunk. This function finds the appropriate
list node and deletes it from the lbl_mgr.lc_list but since
the function uses list_delete_node() the deletion function that is
specified for lbl_mgr.lc_list is not called thus dropping the memory.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>