dma-buf: clarify locking documentation for reservation_object_get_excl

The documentation was misleading, as for a lot of use-cases holding
the RCU read side lock is sufficient.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111165302.25556-2-l.stach@pengutronix.de