io_uring: don't assume uaddr alignment in io_vec_fill_bvec

There is no guaranteed alignment for user pointers. Don't use mask
trickery and adjust the offset by bv_offset.

Cc: stable@vger.kernel.org
Reported-by: David Hildenbrand <david@redhat.com>
Fixes: 9ef4cbbc ("io_uring: add infra for importing vectored reg buffers")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/io-uring/19530391f5c361a026ac9b401ff8e123bde55d98.1750771718.git.asml.silence@gmail.com/


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