io_uring: make offset == -1 consistent with preadv2/pwritev2

io_uring: make offset == -1 consistent with preadv2/pwritev2

[ Upstream commit 0fef948363f62494d779cf9dc3c0a86ea1e5f7cd ]

The man page for io_uring generally claims were consistent with what
preadv2 and pwritev2 accept, but turns out there's a slight discrepancy
in how offset == -1 is handled for pipes/streams. preadv doesn't allow
it, but preadv2 does. This currently causes io_uring to return -EINVAL
if that is attempted, but we should allow that as documented.

This change makes us consistent with preadv2/pwritev2 for just passing
in a NULL ppos for streams if the offset is -1.

Cc: stable@vger.kernel.org # v5.7+
Reported-by: Benedikt Ames <wisp3rwind@posteo.eu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>