Fix fast path in atomicSymlink

[linyinfeng]

Fix #708.

1. If os.Symlink succeeds (err == nil), return nil (err).
2. If os.Symlink fails (err != nil), check if the error indicates that a file or directory already exists (os.IsExist(err)).
   • If the error is unrelated to a pre-existing file or directory, return err as it cannot be handled in atomicSymlink.
   • If the error does indicate a pre-existing file or directory, don't return and handle it with the remaining logic in atomicSymlink.

// Fast path: if newname does not exist yet, we can skip the whole dance
// below.
if err := os.Symlink(oldname, newname); err == nil || !os.IsExist(err) {
    return err
}