SOLARIS: Support VSW_STATS

Specify the VSW_STATS flag to the vfsdef_t structure we give to
Solaris. This turns on statistics that can be retrieved via fsstat(1M)
and allows the fsinfo::: DTrace provider to work with AFS files.

We don't need to actually maintain these statistics; Solaris does that
for us. This flag just signifies that our vfs_t structure is capable
of storing the information. Since we get our vfs_t from Solaris (via
domount(), it gives us a vfs_t when it calls our afs_mount function)
and do not allocate a vfs_t ourselves, we are safe and this is fine to
do.

Change-Id: I356df91ea409245f0c5b1e4ef693ac28ad8f11b2
Reviewed-on: http://gerrit.openafs.org/10679
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>