nfs: stash nfs_subversion reference into nfs_mount_info

That will allow to get rid of passing those references around in
quite a few places.  Moreover, that will allow to merge xdev and
remote file_system_type.

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>