pkg/cover: cap number of addr2line processes
pkg/cover: cap number of addr2line processes

addr2line on a beefy vmlinux takes up to 1.6GB of RAM,
so don't create more than 4 of them.

xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller