zebra: Reduce creation and fix memory leak of frrscripting pointers

zebra: Reduce creation and fix memory leak of frrscripting pointers

There are two issues being addressed:

a) The ZEBRA_ON_RIB_PROCESS_HOOK_CALL script point
was creating a fs pointer per dplane ctx in
rib_process_dplane_results().

b) The fs pointer was not being deleted and directly
leaked.

For (a) Move the creation of the fs to outside
the do while loop.

For (b) At function end ensure that the pointer
is actually deleted.

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