dma-buf: add dma_resv_for_each_fence v3

A simpler version of the iterator to be used when the dma_resv object is
locked.

v2: fix index check here as well
v3: minor coding improvement, some documentation cleanup

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211006123609.2026-1-christian.koenig@amd.com