NFSv4.1: Fix Oopsable condition in server callback races

The slot table hasn't been an array since v3.7. Ensure that we
use nfs4_lookup_slot() to access the slot correctly.

Fixes: 87dda67e ("NFSv4.1: Allow SEQUENCE to resize the slot table...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v3.8+