nfs: flexfilelayout: remove v3-only data server limitation

Flexfilelayout supports data servers which talk NFS v3 and v4.{0,1,2}.
However, this code path is disabled and v3 only servers are accepted.
This change removes this limitation.
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>