io_uring/zcrx: return error from io_zcrx_map_area_*

io_uring/zcrx: return error from io_zcrx_map_area_*

io_zcrx_map_area_*() helpers return the number of processed niovs, which
we use to unroll some of the mappings for user memory areas. It's
unhandy, and dmabuf doesn't care about it. Return an error code instead
and move failure partial unmapping into io_zcrx_map_area_umem().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://lore.kernel.org/r/42668e82be3a84b07ee8fc76d1d6d5ac0f137fe5.1751466461.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>