virSecuritySELinuxRestoreFileLabel: Adjust code pattern

virSecuritySELinuxRestoreFileLabel: Adjust code pattern

Firstly, the following code pattern is harder to follow:

  if (func() < 0) {
      error();
  } else {
      /* success */
  }

We should put 'goto cleanup' into the error branch and move the
else branch one level up.
Secondly, 'rc' should really be named 'ret' because it holds
return value of the function. Not some intermediate value.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>