netbuf fixups: remove mblock before freeing

Change-Id: Idbd3f27802b3346845c93a2c23f82adbd4fd9403
Reviewed-on: http://review.couchbase.org/36085
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>