NFSv4: Ensure decode_compound_hdr() sanity checks the tag

The server is supposed to return the same tag that the client sends in
the outgoing RPC call, but we should still sanity check the length just
in case.

Reported-by:  <rtm@csail.mit.edu>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>