scripts/checkstack.pl: change min_stack to 512 by default

According to Documentation/process/submit-checklist.rst, checkstack does
not point out problems explicitly, but any one function that uses more
than 512 bytes on the stack is a candidate for change, hence it is better
to omit any stack frame sizes smaller than 512 bytes, just change
min_stack to 512 by default.

Link: https://lkml.kernel.org/r/20231219125008.23007-5-yangtiezhu@loongson.cn
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>