fib: fix mpls db label overflow

fib: fix mpls db label overflow

mpls fib DB size was 2^20 instead of intended 2^21.
Therefore large mpls labels caused DB to overflow and write
to other tables or some random objects. Or crash with ASAN.
Sometimes.

Type: fix

Signed-off-by: Dmitry Valter <[email protected]>
Change-Id: I6db65680037a266fe47e8213464a0c16de63c06c