NFSD: copy the whole verifier in nfsd_copy_write_verifier

NFSD: copy the whole verifier in nfsd_copy_write_verifier

BugLink: https://bugs.launchpad.net/bugs/2011425
[ Upstream commit 90d2175572470ba7f55da8447c72ddd4942923c4 ]

Currently, we're only memcpy'ing the first __be32. Ensure we copy into
both words.

Fixes: 91d2e9b56cf5 ("NFSD: Clean up the nfsd_net::nfssvc_boot field")
Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>