util/disk_cache: delete .tmp if target exists

util/disk_cache: delete .tmp if target exists

At the time of target file check, .tmp file is already created and file
lock is held, so we should remove the .tmp, like in other error paths.

With this, piglit no longer leaves large amount of empty .tmp files
behind, which waste directory entries and may interfere with eviction.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>