bgpd: fix dynamic peer graceful restart race condition

bgpd: fix dynamic peer graceful restart race condition

bgp_llgr topotest sometimes fails at step 8:

> topo: STEP 8: 'Check if we can see 172.16.1.2/32 after R4 (dynamic peer) was killed'

R4 neighbor is deleted on R2 because it fails to re-connect:

> 14:33:40.128048 BGP: [HKWM3-ZC5QP] 192.168.3.1 fd -1 went from Established to Clearing
> 14:33:40.128154 BGP: [MJ1TJ-HEE3V] 192.168.3.1(r4) graceful restart timer expired
> 14:33:40.128158 BGP: [ZTA2J-YRKGY] 192.168.3.1(r4) graceful restart stalepath timer stopped
> 14:33:40.128162 BGP: [H917J-25EWN] 192.168.3.1(r4) Long-lived stale timer (IPv4 Unicast) started for 20 sec
> 14:33:40.128168 BGP: [H5X66-NXP9S] 192.168.3.1(r4) Long-lived set stale community (LLGR_STALE) for: 172.16.1.2/32
> 14:33:40.128220 BGP: [H5X66-NXP9S] 192.168.3.1(r4) Long-lived set stale community (LLGR_STALE) for: 192.168.3.0/24
> [...]
> 14:33:41.138869 BGP: [RGGAC-RJ6WG] 192.168.3.1 [Event] Connect failed 111(Connection refused)
> 14:33:41.138906 BGP: [ZWCSR-M7FG9] 192.168.3.1 [FSM] TCP_connection_open_failed (Connect->Active), fd 23
> 14:33:41.138912 BGP: [JA9RP-HSD1K] 192.168.3.1 (dynamic neighbor) deleted (bgp_connect_fail)
> 14:33:41.139126 BGP: [P98A2-2RDFE] 192.168.3.1(r4) graceful restart stalepath timer stopped

af8496af08 ("bgpd: Do not delete BGP dynamic peers if graceful restart
kicks in") forgot to modify bgp_connect_fail()

Do not delete the peer in bgp_connect_fail() if Non-Stop-Forwarding is
in progress.

Fixes: af8496af08 ("bgpd: Do not delete BGP dynamic peers if graceful restart kicks in")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit e446308d76b4ac8f26149db982bf5ed5ac93d5f5)