dma-buf: add missing EXPORT_SYMBOL

The newly added dma_resv_reset_shared_max() is used from an inline
function, so it can appear in drivers as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210604155228.616679-1-christian.koenig@amd.com