bgpd: Shut off keepalives as soon as we shutdown a peer

bgpd: Shut off keepalives as soon as we shutdown a peer

We have this crash:

2019-08-18T07:58:44.831656-04:00 rch2-140-fwK2b bgpd[1791]: %NOTIFICATION: sent to neighbor 10.73.248.8 4/0 (Hold Timer Expired) 0 bytes
2019-08-18T07:58:44.832164-04:00 rch2-140-fwK2b bgpd[1791]: Assertion `!((peer->thread_flags) & ((1 << 0)))' failed in file bgpd.c, line 2173, function peer_delete
2019-08-18T07:58:44.832548-04:00 rch2-140-fwK2b bgpd[1791]: Backtrace for 11 stack frames:
2019-08-18T07:58:44.832942-04:00 rch2-140-fwK2b bgpd[1791]: [bt 0] /usr/lib/libfrr.so.0(zlog_backtrace+0x3a) [0x7f5503c7c31a]
2019-08-18T07:58:44.833311-04:00 rch2-140-fwK2b bgpd[1791]: [bt 1] /usr/lib/libfrr.so.0(_zlog_assert_failed+0x61) [0x7f5503c7c891]
2019-08-18T07:58:44.833684-04:00 rch2-140-fwK2b bgpd[1791]: [bt 2] /usr/lib/frr/bgpd(peer_delete+0x4d5) [0x1432ceea15]
2019-08-18T07:58:44.834095-04:00 rch2-140-fwK2b bgpd[1791]: [bt 3] /usr/lib/frr/bgpd(+0x430e9) [0x1432cfc0e9]
2019-08-18T07:58:44.834479-04:00 rch2-140-fwK2b bgpd[1791]: [bt 4] /usr/lib/frr/bgpd(bgp_event_update+0x121) [0x1432cfe1c1]
2019-08-18T07:58:44.834852-04:00 rch2-140-fwK2b bgpd[1791]: [bt 5] /usr/lib/frr/bgpd(+0x453f1) [0x1432cfe3f1]
2019-08-18T07:58:44.835388-04:00 rch2-140-fwK2b bgpd[1791]: [bt 6] /usr/lib/libfrr.so.0(thread_call+0x60) [0x7f5503c9e3c0]
2019-08-18T07:58:44.835829-04:00 rch2-140-fwK2b bgpd[1791]: [bt 7] /usr/lib/libfrr.so.0(frr_run+0xb8) [0x7f5503c79de8]
2019-08-18T07:58:44.836292-04:00 rch2-140-fwK2b bgpd[1791]: [bt 8] /usr/lib/frr/bgpd(main+0x229) [0x1432ce4a69]
2019-08-18T07:58:44.836729-04:00 rch2-140-fwK2b bgpd[1791]: [bt 9] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7f550271bb45]
2019-08-18T07:58:44.837198-04:00 rch2-140-fwK2b bgpd[1791]: [bt 10] /usr/lib/frr/bgpd(+0x2cefc) [0x1432ce5efc]
2019-08-18T07:58:44.837670-04:00 rch2-140-fwK2b bgpd[1791]: Current thread function (bgp_holdtime_timer), scheduled from file bgp_fsm.c, line 380

This is the code:
	bgp_reads_off(peer);
	bgp_writes_off(peer);
	assert(!CHECK_FLAG(peer->thread_flags, PEER_THREAD_WRITES_ON));
	assert(!CHECK_FLAG(peer->thread_flags, PEER_THREAD_READS_ON));

The line crashing is the first assert.  We know in bgp_writes_off we unset this flag:

void bgp_writes_off(struct peer *peer)
{
        struct frr_pthread *fpt = bgp_pth_io;
        assert(fpt->running);

        thread_cancel_async(fpt->master, &peer->t_write, NULL);
        THREAD_OFF(peer->t_generate_updgrp_packets);

        UNSET_FLAG(peer->thread_flags, PEER_THREAD_WRITES_ON);
}

We also know that the keepalives are not being turned off until we call
bgp_fsm_change_status(peer, Deleted);

later in the function.  We know that the keepalive pthread will
write to individual peers and issue a bgp_write_on(), which sets
this flag.

Modify the code base so that we explicitly turn off the keepalives
immediately before the turning of writes off.

Ticket: CM-26119
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>