rx_multi: free the quantity of memory we allocated

Change-Id: I3a666bb5c3b8b9ab222d1170095e46116fac4229
Reviewed-on: http://gerrit.openafs.org/10266
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>