udmabuf: Ensure to perform cache synchronisation in begin_cpu_udmabuf()

The message of commit 504e2b4ab97a ("dma-buf/udmabuf: skip redundant cpu sync to
fix cacheline EEXIST warning") says:

> The CPU sync at map/unmap time is also redundant for udmabuf:
> begin_cpu_udmabuf() and end_cpu_udmabuf() already perform explicit
> cache synchronization via dma_sync_sgtable_for_cpu/device() when CPU
> access is requested through the dma-buf interface.

This, however, does not apply to the first time begin_cpu_udmabuf() is
called on an udmabuf, in which case the implementation previously relied on
get_sg_table() to perform the cache synchronisation.

Ensure to call dma_sync_sgtable_for_cpu() in that case as well.

Fixes: 504e2b4ab97a ("dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning")
Signed-off-by: Robert Mader <robert.mader@collabora.com>
Reviewed-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patch.msgid.link/20260627105725.9083-1-robert.mader@collabora.com

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