BACKPORT: dma-buf: keep the signaling time of merged fences v3

Some Android CTS is testing if the signaling time keeps consistent
during merges.

v2: use the current time if the fence is still in the signaling path and
the timestamp not yet available.
v3: improve comment, fix one more case to use the correct timestamp

Bug: 379008999
Change-Id: Ia29fbb61d7a2a3c32fecfc2dbf30fcb39eeff38f
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630120041.109216-1-christian.koenig@amd.com


(cherry picked from commit f781f661)
[kiyoshi: Resolved conflict regarding drivers/dma-buf/dma-fence-unwrap.c
Added alternative function to avoid changing
dma_fence_allocate_private_stub which is already exported]
Signed-off-by: Kiyoshi Konno <konno.kiyoshi@sharp.corp-partner.google.com>
[TJ: Add dma-fence-unwrap.o to Makefile now that it has been adapted to
work with our new dma_fence_allocate_private_stub_with_timestamp
function in an ABI-stable way.]
Signed-off-by: T.J. Mercier <tjmercier@google.com>