netfilter: ebtables: clean up initialization of buf

buf is initialized to buf_start and then set on the next statement
to buf_start + offsets[i].  Clean this up to just initialize buf
to buf_start + offsets[i] to clean up the clang build warning:
"Value stored to 'buf' during its initialization is never read"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>