rx: Refactor MaxMTU error checking

rx: Refactor MaxMTU error checking

The error checking on the rxMaxMTU parameter was done individually by
every server that sets it, using "internal" RX #defines to do so.
Instead, do the error checking within the function that actually sets
the MTU, reducing both the amount of code duplication, and the amount
of RX knowledge held within the servers.

Change-Id: Ic2cdd9425d5344a5137f76f66f711f4dee91a7b6
Reviewed-on: http://gerrit.openafs.org/6091
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>