nat: fix potential out-of-bound worker array index

nat: fix potential out-of-bound worker array index

In several NAT submodules, the number of available ports (0xffff - 1024)
may not be divisible by the number of workers, so port_per_thread is
determined by integer division, which is the floor of the quotient.
Later when a worker index is needed, dividing the port with port_per_thread
may yield an out-of-bound array index into the workers array.

As an example, assume 2 workers are configured, then port_per_thread
will be (0xffff - 1024) / 2, which is 32255. When we compute a worker
index with port 0xffff, we get (0xffff - 1024) / 32255, which is 2,
but since we only have 2 workers, only 0 and 1 are valid indices.

This patch fixes the problem by adding a modulo at the end of the division.

Type: fix
Signed-off-by: Jing Peng <pj.hades@gmail.com>
Change-Id: Ieae3d5faf716410422610484a68222f1c957f3f8