kmac: fix customistation string overflow bug

Previously there was an off by two error allowing a stack buffer overrun.
Avoided this by allocating a correct sized buffer on the stack.  A side effect
is that the maximum size of the customisation string can be increased.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/14810)
