OSPFD: Update Segment Routing implementation

OSPFD: Update Segment Routing implementation

 - ospf_ext.c: Correct deferred pointer raised by valgrind
 - ospf_sr.c: Correct deffered pointer raised by valgrind. Modify
Segment Routing shutdown. This is due to the fact that RI LSA
4.0.0.0 is flush prior to 7.0.0.X & 8.0.0.X LSA. This trigger
SR-Node deletion which also remove all Extended Link / Prefix
leaving them unavailable when there are referenced by 7.0.0.X
& 8.0.0.X LSA flushing.
 - doc/OSPF-SR.rst: Correct Restructured Text syntax and add
Configuration example as well as Credit section

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>