nfs: RPC_MAX_AUTH_SIZE is in bytes

The units of RPC_MAX_AUTH_SIZE is bytes, not 4-byte words.  This causes
the client to request a larger-than-necessary session replay slot size.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>