Unix CM: Make rootVolume array big enough

Unix CM: Make rootVolume array big enough

In afs_CheckRootVolume, the local rootVolumeName array needs to
be large enough to hold the contents of the global
afs_rootVolumeName string, which is 64 characters long. Fix our
local array to be the same length by using a new defined constant
MAXROOTVOLNAMELEN.

Caught by coverity (#985758)

Change-Id: I4c926b94efb40d7107e2d7160ade0ba8b381004e
Reviewed-on: http://gerrit.openafs.org/9348
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>