Implement OpenBSD 5.6 Errata 001:

Incorrect RX ring computation leads to panics under load with bge(4),
em(4) and ix(4).

Revert part of the if_rxr diff that incorrectly moves RX ring tail
index update code from the buf_get success path to the do it all
the time code path.  Tested by millert;  ok dlg, deraadt