Set initial values for nfsstatfs in the NFSv4 client.

The AmazonEFS NFSv4.1 server does not support the FILES_FREE and FILES_TOTAL
attributes. As such, an NFSv4.1 mount to the server would return garbage
for these values. This patch initializes the fields of the nfsstatfs structure,
so that "df" and friends will at least return consistent bogus values.
This patch should have effect when mounting other NFSv4.1 servers.

Reported by: cperciva
MFC after: 2 weeks