fix potential deadlock in create_one_file

It can happen if the temporary file already exists (i.e. after a panic
and reboot).

Signed-off-by: Alex Riesen <redacted>

Alex Riesen [off-list ref] writes:

It can happen if the temporary file already exists (i.e. after a panic
and reboot).

We use pid here and using somebody else's pid feels wrong but
that is only for collision avoidance, mktemp(3) is no better,
and neither tmpfile(3) nor mkstemp(3) are usable for directories
anyway, so your patch as is would be the best solution.

Thanks.