tc: nat: Fix mask calculation

tc: nat: Fix mask calculation

In parse_nat_args the network mask is calculated as

        sel->mask = htonl(~0u << (32 - addr.bitlen));

According to  ISO/IEC 9899:TC3 6.5.7 Bitwise shift operators:
The integer promotions are performed on each of the operands.
The type of the result is that of the promoted left operand.
If the value of the right operand is negative or is greater
than or equal to the width of the promoted left operand,
the behavior is undefined

Specifically this means that the mask is undefined for
addr.bitlen = 0
On x86_64 the result is 0xffffffff, on armv7l it is 0.

Promoting the left operand of the shift operator solves this issue.

Signed-off-by: Torben Nielsen <torben.nielsen@prevas.dk>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>