NFS: Clean up find_nfs_version()

It's good practice to check the return value of request_module() to see
if the module has been found. It's also a little easier to follow the
code if __find_nfs_version() doesn't attempt to convert NULL pointers
into -EPROTONOSUPPORT.

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