NFSv4: Fix a typo in nfs41_sequence_process

We want to compare the slot_id to the highest slot number advertised by the
server.

Fixes: 3be0f80b ("NFSv4.1: Fix up replays of interrupted requests")
Cc: stable@vger.kernel.org # 4.15+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>