Merge 9bb48c82 ("tty: implement write_iter") into tty-linus

We want the single "splice/sendfile to a tty" regression fix into
tty-linus so it can get into 5.11-final, while the larger patch series
fixing "splice/sendfile from a tty" should wait for 5.12-rc1 so that we
get more testing.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>