accel/amdxdna: refuse to flush an imported BO

SYNC_BO clflushes an imported BO's scatterlist. An importer may not do
that: the memory belongs to the exporter, and dma-buf gives the importer
no interface to ask for maintenance on it. Refuse the request instead.

is_import_bo() is (obj)->attach, which covers more than foreign buffers.
A userptr BO arrives through a ubuf, and on a carveout device every share
BO and the device heap arrive through a cbuf, so SYNC_BO answers
-EOPNOTSUPP for those too, including the AMDXDNA_BO_DEV path that flushes
through its heap.

Only the ubuf case gives up maintenance it was getting: on a 64 MiB
userptr BO a 4 KiB sync and a full sync both cost 659 us, this arm having
ignored the range. amdxdna_cbuf_map() fills in only the DMA address and
length, so drm_clflush_sg() already walks zero pages on carveout memory.
Userspace maintains these through the mapping it already holds, as XRT's
buffer::sync() does unless it is told to sync through the driver.

Fixes: dbc8fd7a03cb ("accel/amdxdna: Add expandable device heap support")
Reported-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/dri-devel/a505f9e5-b416-43e9-934d-c5c29b8a70e9@amd.com/
Suggested-by: Lizhi Hou <lizhi.hou@amd.com>
Signed-off-by: Taimuraz Kaitmazov <taimuraz@kaitmazov.com>
Reviewed-by: Lizhi Hou <lizhi.hou@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260819224458.257346-5-taimuraz@kaitmazov.com

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