zebra: dead code (Coverity 1465497)

zebra: dead code (Coverity 1465497)

The condition in the do/while is always false because 'return_nsid' cannot
reach the end of the loop with 'return_nsid' having a different value than
NS_UNKNOWN. Because of that, the condition can be replaced with 0 (false).
Also, the loop can be removed because the two assignments made at the end
of the loop before the condition check are not used (detected via Clang,
afterwards).

Signed-off-by: F. Aragon <paco@voltanet.io>