io_uring: clean __io_import_iovec()

Apparently, implicit 0 to NULL conversion with ERR_PTR is not
recommended and makes some tooling like Smatch to complain. Handle it
explicitly, compilers are perfectly capable to optimise it out.

Link: https://lore.kernel.org/all/20211108134937.GA2863@kili/


Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5c6ed369ad95075dab345df679f8677b8fe66656.1637524285.git.asml.silence@gmail.com


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