nfsd: Fix a memory leak in an error handling path

If this memdup_user() call fails, the memory allocated in a previous call
a few lines above should be freed. Otherwise it leaks.

Fixes: 6ee95d1c ("nfsd: add support for upcall version 2")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>