openvswitch: meter: Use 64-bit arithmetic instead of 32-bit

Add suffix LL to constant 1000 in order to give the compiler
complete information about the proper arithmetic to use. Notice
that this constant is used in a context that expects an expression
of type long long int (64 bits, signed).

The expression (band->burst_size + band->rate) * 1000 is currently
being evaluated using 32-bit arithmetic.

Addresses-Coverity-ID: 1461563 ("Unintentional integer overflow")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>