io_uring: don't use ERR_PTR for user pointers

ERR_PTR abuses the high bits of a pointer to transport error information.
This is only safe for kernel pointers and not user pointers.  Fix
io_buffer_select and its helpers to just return NULL for failure and get
rid of this abuse.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220518084005.3255380-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>