NFSD: Trace NFSv4 COMPOUND tags

The Linux NFSv4 client implementation does not use COMPOUND tags,
but the Solaris and MacOS implementations do, and so does pynfs.
Record these eye-catchers in the server's trace buffer to annotate
client requests while troubleshooting.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>