afs: Increase NUSERS to 2k

afs: Increase NUSERS to 2k

The current value of NUSERS (16) is very small when the client has a
large number of PAGs. When we have a few thousand (or even just a few
hundred), the length of the unixuser hash chains can be come long
enough to cause noticeable differences in performance. Looking up
unixuser structs is a very common code path, since we need to do this
for pretty much any file access that's not already cached at the VFS
layer.

To alleviate this, increase NUSERS to 2k. This is certainly not
necessary on all systems (such as those that never use authentication,
or only have a few PAGs), but the overhead is a very small constant
amount of memory.

Change-Id: I17b32f192656db9ffc97ac47b0923a02803977a3
Reviewed-on: https://gerrit.openafs.org/14959
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>