vtysh: [7.0] exit-vrf cli is broken

vtysh: [7.0] exit-vrf cli is broken

exit-vrf is not working any more, so anything configured after executing this command are still configured under that VRF context.
For example: Below configuration

vrf VRF_A
ip route 11.11.11.11/32 Null0
exit-vrf
ip route 21.21.21.21/32 Null0

results in ...

vrf VRF_A
 ip route 11.11.11.11/32 Null0
 ip route 21.21.21.21/32 Null0
 exit-vrf
!

It should have been

!
ip route 21.21.21.21/32 Null0
!
vrf VRF_A
 ip route 11.11.11.11/32 Null0
 exit-vrf
!

Porting https://github.com/FRRouting/frr/pull/4725 from FRR master.

Signed-off-by: Sri Mohana Singamsetty <msingamsetty@vmware.com>