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

All dma-buf functions has been moved to dynamic locking specification
The dma_buf_mmap_internal() was missed out by accident. Take reservation
lock around file mapping operation to adhere the common locking convention.

Reported-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221110201349.351294-2-dmitry.osipenko@collabora.com