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: 52791eeec1d9 ("dma-buf: rename reservation_object to dma_resv')
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/driver-api/dma-buf.rst

···
 Reservation Objects
 -------------------
 
-.. kernel-doc:: drivers/dma-buf/reservation.c
+.. kernel-doc:: drivers/dma-buf/dma-resv.c
    :doc: Reservation Object Overview
 
-.. kernel-doc:: drivers/dma-buf/reservation.c
+.. kernel-doc:: drivers/dma-buf/dma-resv.c
    :export:
 
-.. kernel-doc:: include/linux/reservation.h
+.. kernel-doc:: include/linux/dma-resv.h
    :internal:
 
 DMA Fences