allocator/dumb: Pass correct pitch in `AsDmabuf::export` #3262

Job	Run time
format	13s
clippy-check	2m 34s
check-msrv	4m 5s
check-minimal	2m 7s
smithay-tests	1m 29s
smithay-tests (backend_winit)	3m 13s
smithay-tests (backend_drm)	1m 41s
smithay-tests (backend_gbm)	1m 43s
smithay-tests (backend_egl)	1m 42s
smithay-tests (backend_libinput)	2m 38s
smithay-tests (backend_udev)	1m 58s
smithay-tests (backend_session)	1m 34s
smithay-tests (backend_session_libseat)	1m 38s
smithay-tests (backend_vulkan)	1m 56s
smithay-tests (backend_x11)	2m 51s
smithay-tests (desktop)	1m 50s
smithay-tests (renderer_gl)	1m 46s
smithay-tests (renderer_glow)	3m 1s
smithay-tests (renderer_multi)	2m 3s
smithay-tests (wayland_frontend)	2m 21s
smithay-tests (xwayland)	2m 34s
smithay-tests (default)	4m 46s
smithay-tests (test_all_features)	5m 6s
smallvil	1m 29s
Documentation on Github Pages	1m 43s
anvil-tests	1m 16s
anvil-tests (egl)	1m 27s
anvil-tests (winit)	2m 20s
anvil-tests (udev)	2m 6s
anvil-tests (x11)	2m 42s
anvil-tests (default)	2m 48s
anvil-tests (test_all_features)	2m 22s
WLCS: Core tests	2m 52s
WLCS: Output tests	2m 6s
WLCS: Pointer input tests	2m 19s
WLCS: Bad Buffer Test	2m 9s
	1h 22m 28s

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