afs: Fix copy_file_range()

The prevention of splice-write without explicit ops made the
copy_file_write() syscall to an afs file (as done by the generic/112
xfstest) fail with EINVAL.

Fix by using iter_file_splice_write() for afs.

Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>