netfilter: xtables: Use strscpy() instead of strscpy_pad()

kzalloc() already zero-initializes the destination buffer, making
strscpy() sufficient for safely copying the name. The additional NUL-
padding performed by strscpy_pad() is unnecessary.

The size parameter is optional, and strscpy() automatically determines
the size of the destination buffer using sizeof() if the argument is
omitted. This makes the explicit sizeof() call unnecessary; remove it.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>