NFSD: Refactor nfsd4_do_copy()

[ Upstream commit 3b7bf593 ]

Refactor: Now that nfsd4_do_copy() no longer calls the cleanup
helpers, plumb the use of struct file pointers all the way down to
_nfsd_copy_file_range().

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