io_uring: fix explicit async read/write mapping for large segments
If we exceed UIO_FASTIOV, we don't handle the transition correctly
between an allocated vec for requests that are queued with IOSQE_ASYNC.
Store the iovec appropriately and re-set it in the iter iov in case
it changed.
Fixes: 2f0408fc0be9 ("io_uring: retain iov_iter state over io_read/io_write calls") Reported-by: Nick Hill <nick@nickhill.org> Tested-by: Norman Maurer <norman.maurer@googlemail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>