pkg/osutil: fix IsExist
pkg/osutil: fix IsExist

Currently it returns true for "/some/existing/file/foo"
because that returns ENOTDIR.

dotweiba/syzkaller,zoulasc/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller,hyz0906/syzkaller,google/syzkaller,google/syzkaller,dotweiba/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,hyz0906/syzkaller,google/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,xairy/syzkaller,hyz0906/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,zoulasc/syzkaller,google/syzkaller,xairy/syzkaller,hyz0906/syzkaller