vol: reset nextVnodeUnique when uniquifier rolls over

The on disk uniquifier counter is set to 200 more than the current
nextVnodeUnique counter when the volume information is updated to disk. When
the nextVnodeUnique is near UINT32_MAX, then the uniquifier counter rolls
over.  This can happen during a volume header update due to
VBumpVolumeUsage_r().

With this change, the nextVnodeUnique customer is reset to 2 and the
uniquifier is reset to 202 when a roll over occurs. (uniquifier of 1 is
reserved for the root vnode.)

With this change, the number of possible uniquifier numbers is limited to
200 less than UINT32_MAX.

The following shows a series of vnode creation/deletions to illustrate
the uniquifier rollover before this commit:

fid = 536870918.4.4294967114, nextVnodeUnique = 4294967115, uniquifier = 4294967295
fid = 536870918.4.4294967115, nextVnodeUnique = 4294967116, uniquifier = 4294967295
fid = 536870918.4.4294967116, nextVnodeUnique = 4294967117, uniquifier = 21
fid = 536870918.4.4294967117, nextVnodeUnique = 4294967118, uniquifier = 22

and after this commit:

fid = 536870918.4.4294967115, nextVnodeUnique = 4294967116, uniquifier = 4294967295
fid = 536870918.4.4294967116, nextVnodeUnique = 2, uniquifier = 202
fid = 536870918.4.2, nextVnodeUnique = 3, uniquifier = 202
fid = 536870918.4.3, nextVnodeUnique = 4, uniquifier = 202

Reviewed-on: http://gerrit.openafs.org/10617
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 64d7715c0247734731ef4cc8be5de32ee7c4a1f6)

Change-Id: Ieead6921f8ebca436cbcaecbf7711236c6cdf0e9
Reviewed-on: http://gerrit.openafs.org/10847
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>