NFS: Convert the NFS module list into an array

Using a linked list here seems unnecessarily complex, especially since
possible index values are '2', '3', and '4'. Let's just use an array for
direct access.

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