Partition count is of a type size_t but we use std::plus<int>
to reduce values of partition count in various column families.
This patch changes the argument of std::plus to the right type.
Using std::plus<int> for size_t compiles but does not work as expected.
For example plus<int>(2147483648LL, 1LL) = -2147483647 while the code
would probably want 2147483649.
Fixes#9090
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
Closes#9074
90a607e844