vlserver: fix vlentryread() for old vldb formats

vlserver: fix vlentryread() for old vldb formats

When we're using old format compatibility, use OMAXNSERVERS for the
array lengths instead of MAXNSERVERS.  Otherwise we'll try to copy more
data than we've read.

Detected by gcc8 as:

vlutils.c:183:2: error: ‘memcpy’ forming offset [149, 151] is out of the bounds [0, 148] of object ‘tentry’ with type ‘struct vlentry’ [-Werror=array-bounds]
  memcpy(nbufp->serverFlags, oep->serverFlags, NMAXNSERVERS);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
vlutils.c:141:26: note: ‘tentry’ declared here
     struct vlentry *oep, tentry;
                          ^~~~~~

Change-Id: Ie720ca037c5a8bd6aaff5b6d5348161e0175b23b
Reviewed-on: https://gerrit.openafs.org/13465
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>