dma-buf: rename dma_resv_get_excl_rcu to _unlocked

That describes much better what the function is doing here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602111714.212426-6-christian.koenig@amd.com