vlserver: increase the max lwp threads

Increase the maximum LWP threads allowed from 16 to 64. Increasing the number
of LWP threads can reduce the number of calls waiting for threads on
busy vlservers.

Change-Id: I1e95366aad8fe4d275794603d2af07c799cd7048
Reviewed-on: http://gerrit.openafs.org/9706
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>