io_uring: fix provided buffer import

io_import_iovec uses the s pointer, but this was changed immediately
after the iovec was re-imported and so it was imported into the wrong
place.

Change the ordering.

Fixes: 2be2eb02

 ("io_uring: ensure reads re-import for selected buffers")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220630132006.2825668-1-dylany@fb.com
[axboe: ensure we don't half-import as well]
Signed-off-by: Jens Axboe <axboe@kernel.dk>