afs: Use named constants for mvstat

afs: Use named constants for mvstat

Currently the vcache 'mvstat' field is assigned three magic values: 0
for normal files and directories, 1 for mountpoint objects, and 2 for
volume root dirs. These values are clearly defined in comments, but
everywhere we actually assign or compare these values, we use the bare
numbers.

Stop this nonsense and use named constants, to make the code less
inscrutable.

Change-Id: Ic1b133109d619b70317141431f163e552bafd109
Reviewed-on: http://gerrit.openafs.org/11747
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>