rx mtu discovery constrainment code

rx mtu discovery constrainment code

this code makes us work in the face of a sub1500 mtu network.
a subsequent commit is needed to make it more effective: attempts
to grow the mtu must be scheduled so we aren't forced to heavily
brute-force on failure to discover the exact mtu immediately;
for performance, we do want to grow to the real mtu.

Change-Id: If876c134efd4b0952035193ef00a6e780f7adc64
Reviewed-on: http://gerrit.openafs.org/2069
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>