io_uring/advise: support 64-bit lengths

The existing fadvise/madvise support only supports 32-bit lengths. Add
support for 64-bit lengths, enabled by the application setting sqe->off
rather than sqe->len for the length. If sqe->len is set, then that is
used as the 32-bit length. If sqe->len is zero, then sqe->off is read
for full 64-bit support.

Older kernels will return -EINVAL if 64-bit support isn't available.

Fixes: 4840e418 ("io_uring: add IORING_OP_FADVISE")
Fixes: c1ca757b

 ("io_uring: add IORING_OP_MADVISE")
Reported-by: Stefan <source@s.muenzel.net>
Signed-off-by: Jens Axboe <axboe@kernel.dk>