NFSv4.2: fix copy stateid copying for the async copy

This patch fixes Dan Carpenter's report that the static checker
found a problem where memcpy() was copying into too small of a buffer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e0639dc5 ("NFSD introduce async copy feature")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Dai Ngo <dai.ngo@oracle.com>