xfrm: Remove unnecessary strscpy_pad() size arguments

If the destination buffer has a fixed length, strscpy_pad()
automatically determines its size using sizeof() when the argument is
omitted. This makes the explicit sizeof() calls unnecessary - remove
them.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>