NFS: Disable READ_PLUS by default

We've been seeing failures with xfstests generic/091 and generic/263
when using READ_PLUS. I've made some progress on these issues, and the
tests fail later on but still don't pass. Let's disable READ_PLUS by
default until we can work out what is going on.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>