dma-buf: add dma_resv selftest v4

Just exercising a very minor subset of the functionality, but already
proven useful.

v2: add missing locking
v3: some more cleanup and consolidation, add unlocked test as well
v4: add a dma_resv_get_fences selftest

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v3)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v3)
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-4-christian.koenig@amd.com