nfsd4: disallow SEEK with special stateids

If the client uses a special stateid then we'll pass a NULL file to
vfs_llseek.

Fixes: 24bab491 " NFSD: Implement SEEK"
Cc: Anna Schumaker <Anna.Schumaker@Netapp.com>
Cc: stable@vger.kernel.org
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>