nfs: Fix number of fields for NFSv4

nfs: Fix number of fields for NFSv4

The previous buffer of 64 fields was too short for the current number of
72+2 fields in NFS 4.2, causing the message

  nfs plugin: Unexpected number of fields for NFSv4 server statistics: 62.

This change, originally proposed in [1], fixes that issue in a
forward-compatible manner by reusing the already known number of fields
from the static arrays.

[1]: https://github.com/collectd/collectd/issues/2076#issuecomment-350466628