io_uring/zcrx: disallow user selected dmabuf offset and size

zcrx shouldn't be so frivolous about cutting a dmabuf sgtable and taking
a subrange into it, the dmabuf layer might be not expecting that. It
shouldn't be a problem for now, but since the zcrx dmabuf support is new
and there shouldn't be any real users, let's play safe and reject user
provided ranges into dmabufs. Also, it shouldn't be needed as userspace
should size them appropriately.

Fixes: a5c98e94 ("io_uring/zcrx: dmabuf backed zerocopy receive")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/be899f1afed32053eb2e2079d0da241514674aca.1752443579.git.asml.silence@gmail.com


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