kernfs: implement ->write_iter

Switch kernfs to implement the write_iter method instead of plain old
write to prepare to supporting splice and sendfile again.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210120204631.274206-3-hch@lst.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>