docs/driver-api: Catch up with dma_buf file-name changes

drivers/dma_buf/reservation.c was renamed to dma-resv.c (and
include/linux/reservation.h to dma-resv.h), but the documentation was not
updated to match, leading to these build errors:

  Error: Cannot open file ./drivers/dma-buf/reservation.c
  Error: Cannot open file ./drivers/dma-buf/reservation.c
  Error: Cannot open file ./drivers/dma-buf/reservation.c
  Error: Cannot open file ./include/linux/reservation.h
  Error: Cannot open file ./include/linux/reservation.h

Update the documentation and make the world happy again.

Fixes: 52791eee ("dma-buf: rename reservation_object to dma_resv')
Signed-off-by: Jonathan Corbet <corbet@lwn.net>