NFSD: Remove kmalloc from nfsd4_do_async_copy()

[ Upstream commit ad1e46c9 ]

Instead of manufacturing a phony struct nfsd_file, pass the
struct file returned by nfs42_ssc_open() directly to
nfsd4_do_copy().

[ cel: adjusted to apply to v5.10.y ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>