fib: fix possible integer overflow

fib: fix possible integer overflow

This commit fixes possible integer overflow for
prev_idx in build_common_root() CID 350596
and
tbl8_idx in write_edge() CID 350597

Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
overflow_before_widen: Potentially overflowing expression tbl8_idx * 256
with type int (32 bits, signed) is evaluated using 32-bit arithmetic,
and then used in a context that expects an expression of
type uint64_t (64 bits, unsigned).

Coverity issue: 350596, 350597
Fixes: c3e12e0f0354 ("fib: add dataplane algorithm for IPv6")
Cc: stable@dpdk.org
Signed-off-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>