Use reasonably sized stack variable for signature storage

[evelikov]

as you mentioned in the commit message, in future we could do: check the total_len in the normal case (i.e. dump it by callinrg this for all modules), then use DECLARE_STRBUF_WITH_STACK() with a reasonable size.

Originally posted by @lucasdemarchi in #427 (comment)

[evelikov]

Should check across distros and kernels - ideally some older and newer.