dma-buf: add some lockdep asserts to the reservation object implementation

This adds lockdep asserts to the reservation functions which state in their
documentation that obj->lock must be held. Allows builds with PROVE_LOCKING
enabled to check that the locking requirements are met.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111165302.25556-1-l.stach@pengutronix.de