io_uring/zcrx: cache fallback availability in zcrx ctx

io_uring/zcrx: cache fallback availability in zcrx ctx

Store a flag in struct io_zcrx_ifq telling if the backing memory is
normal page or dmabuf based. It was looking it up from the area, however
it logically allocates from the zcrx ctx and not a particular area, and
once we add more than one area it'll become a mess.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/65e75408a7758fe7e60fae89b7a8d5ae4857f515.1774261953.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>