nfsd: Fix a memory scribble in the callback channel

The offset of the entry in struct rpc_version has to match the version
number.

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Fixes: 1c5876dd ("sunrpc: move p_count out of struct rpc_procinfo")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>