zebra: Install/Update success caused flags to be cleared for lsp

zebra: Install/Update success caused flags to be cleared for lsp

When the kernel installs/updates a lsp the return code for
success is a 0.  The code was interpreting the 0 as a failure
case for the Install/Update code paths.  This caused upon
a true deletion zebra loosing knowledge of the lsp
but the kernel still had it installed.

Failure:
mpls label bind 10.50.4.11/32 4444
!
line vty
!
end
robot.cumulusnetworks.com# conf t
robot.cumulusnetworks.com(config)# no mpls lsp 6666 10.50.11.1 3933
robot.cumulusnetworks.com(config)# exit
robot.cumulusnetworks.com# show mpls table 6666
robot.cumulusnetworks.com# exit
sharpd@robot ~/frr4> ip -f mpls route show
6666 as to 3933 via inet 10.50.11.1 dev enp0s10 proto static

With Fix:
sharpd@robot ~/frr4> ip -f mpls route show
6666 as to 3933 via inet 10.50.11.1 dev enp0s10 proto static
sharpd@robot ~/frr4> sudo vtysh

Hello, this is FRRouting (version 3.1-dev).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

robot.cumulusnetworks.com# conf t
robot.cumulusnetworks.com(config)# no mpls lsp 6666 10.50.11.1 3933
robot.cumulusnetworks.com(config)# end
robot.cumulusnetworks.com# show mpls table 6666
robot.cumulusnetworks.com# exit
sharpd@robot ~/frr4> ip -f mpls route show
sharpd@robot ~/frr4>

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