nfs: Fix kerneldoc warning shown up by W=1

Fix a kerneldoc warning in nfs due to documentation for a parameter that
isn't present.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Anna Schumaker <anna.schumaker@netapp.com>
cc: Mauro Carvalho Chehab <mchehab@kernel.org>
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/163214005516.2945267.7000234432243167892.stgit@warthog.procyon.org.uk/ # rfc v1
Link: https://lore.kernel.org/r/163281899704.2790286.9177774252843775348.stgit@warthog.procyon.org.uk/ # rfc v2