ipv6: remove redundant store to value after addition

ipv6: remove redundant store to value after addition

There is no need to store the result of the addition back to variable count
after the addition. The store is redundant, replace += with just +

Cleans up clang scan build warning:
warning: Although the value stored to 'count' is used in the enclosing
expression, the value is never actually read from 'count'

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220628145406.183527-1-colin.i.king@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>