nfs41: drop dependency between flexfiles layout driver and NFSv3 modules

The flexfiles layout driver depends on NFSv3 module as data servers
might be configure to provide nfsv3 only.

Disabling the nfsv3 protocol completely disables the flexfiles layout driver,
however, the data server still might support v4.1 protocol. Thus the strond
couling betwwen flexfiles and nfsv3 modules should be relaxed, as layout driver
will return UNSUPPORTED if not matching protocol is found.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>