/dev/mem: nowait zero/null ops

Make read_iter_zero() to honor IOCB_NOWAIT, so /dev/zero can be
advertised as FMODE_NOWAIT. It's useful for io_uring, which needs it to
apply certain optimisations when doing I/O against the device.

Set FMODE_NOWAIT for /dev/null as well, it never waits and therefore
trivially meets the criteria.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f11090f97ddc2b2ce49ea1211258658ddfbc5563.1631127867.git.asml.silence@gmail.com


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