linux/sched_migrate_cpu: work around clang -Wcomma warning
linux/sched_migrate_cpu: work around clang -Wcomma warning

clang does not like the double-initialisation of the for loop:

    sched_migrate_cpu.c:106:15: error: possible misuse of comma operator
    here [-Werror,-Wcomma]
        for (i = 0, cpu_num = 0; i < count && cpu_num <= 8 * setsize; cpu_num++) {
                  ^
    sched_migrate_cpu.c:106:10: note: cast expression to void to silence
    warning
        for (i = 0, cpu_num = 0; i < count && cpu_num <= 8 * setsize; cpu_num++) {
             ^~~~~
             (void)( )

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