afs: Log weird 'size' fetchdata errors

afs: Log weird 'size' fetchdata errors

There are a couple of situations that should never happen when issuing
a fetchdata, but cause errors when they do:

 - The fileserver responds with more than 2^32 bytes of data
 - The fileserver responds with more data than requested (but still
   smaller than 2^32)

While these should normally never be encountered, it can be very
confusing when they do, since they cause file fetches to fail. To give
the user or investigating developer some hope of figuring out what is
going on, at least log a warning in these situations, to at least
indicate this is the area in which something is breaking.

Only log these once, in case something causes these conditions to be
hit, e.g., every fetch. Once is at least enough to say this is
happening.

[mmeffie@sinenomine.net remove unneeded casts in afs_warn args and
explicit static initializers.]

Reviewed-on: http://gerrit.openafs.org/11830
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 5fbf45b56298aa5a93cf9015f2d6346c7a0f615c)

Change-Id: I2f15255f33f44bef038ac9926d1ed47eca73d89a
Reviewed-on: https://gerrit.openafs.org/12215
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>