vipw: Set correct pattern for fmkomstemp

vipw: Set correct pattern for fmkomstemp

The fmkomstemp call requires a suffix of XXXXXX for correct operation.
Do so in TCB case as well.

Note: If something fails and the file resides in this directory, it
could be interpreted as a username. Use the ',' character as an illegal
character to prevent shadow tools from erroneously accessing this file
and assuming that the user actually exists.

Fixes: a5b3d56e2902 (2026-01-09; "vipw: Use fmkomstemp for temporary file")
Reported-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>