NFSv4.2: Change the default KConfig value for READ_PLUS

Now that we've worked out performance issues and have a server patch
addressing the failed xfstests, we can safely enable this feature by
default.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>