habanalabs: fix dmabuf to export only required size

This patch fixes a bug that was found in the dmabuf flow.
Bug description as found on Gaudi2 device:
1. User allocates 4MB of device memory
    - Note that although the allocation size was 4MB the HMMU allocated
      a full page of 768MB to back the request.
    - The user gets a memory handle that points to a single page (768MB)
    - Mapping the handle, the user gets virtual address to the start of
      the page.
2. User exports the buffer
3. User registers the exported buffer in the importer. This flow has
   a callback to the exporter which in turn converts the phys_page_pack
   to an SG list for the importer. This SG list is of single entry of
   size 768MB. However, the size that was passed to the importer was
   only 4MB.

The solution for this is to make sure the importer gets exposure only
to the exported size.

This will be done by fixing the SG created by the exporter to be of
the total size of the actual exported memory requested by the user.

Signed-off-by: Ohad Sharabi <osharabi@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>