tombstoned: make missing O_TMPFILE workaround actually work around.

We can't actually link an unlinked file back onto disk if it wasn't
opened with O_TMPFILE. Switch to using a temporary filename instead.

Bug: http://b/77729983
Test: agampe
Change-Id: I1970497114f0056065a1ba65f6358f08b51ec551
(cherry picked from commit f5974aed)