Fix: out of bounds access of kernel channel padding

The padding of the old ABI is bigger than the new one so we use the size
of the new padding size for the memcpy since it will always be smaller.

In kernctl_create_channel: Out-of-bounds access to a buffer (CWE-119).
In kernctl_open_metadata: Out-of-bounds access to a buffer (CWE-119).

Issue 1019925 and 1019924 of coverity scan.

Signed-off-by: David Goulet <dgoulet@efficios.com>