taskset: use iterate_on_dir()

function                                             old     new   delta
iter                                                   -      27     +27
process_pid_str                                      854     856      +2
taskset_main                                         181     135     -46
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(add/remove: 1/0 grow/shrink: 1/1 up/down: 29/-46)            Total: -17 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed