dma-buf: Move dma_buf_vmap() to dynamic locking specification

Move dma_buf_vmap/vunmap() functions to the dynamic locking
specification by asserting that the reservation lock is held.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221017172229.42269-16-dmitry.osipenko@collabora.com