nfsd: fix using the correct variable for sizeof()

[ Upstream commit 4fc5f534 ]

While the original code is valid, it is not the obvious choice for the
sizeof() call and in preparation to limit the scope of the list iterator
variable the sizeof should be changed to the size of the destination.

Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>