Fix the pNFS server's reporting of disk space usage for the "#<path>" case.

The pNFS server would report the total disk space used and free for all
of the DSs, even when certain DSs are assigned to the file system via
the "#<path>" suffix used in the "nfsd -p" option argument.
This patch fixes this case. It only reports usage for the file system
that the argument vnode resides on. This is consistent with the non-pNFS
NFSv4 server. In NFSv4 it is possible to have subtrees on other file
systems, but these are not included in the usage information for NFSv4.

Approved by: re (gjb)