xfrm: Zero padding when dumping algos and encap

When copying data to user-space we should ensure that only valid
data is copied over.  Padding in structures may be filled with
random (possibly sensitve) data and should never be given directly
to user-space.

This patch fixes the copying of xfrm algorithms and the encap
template in xfrm_user so that padding is zeroed.

Reported-by:  <syzbot+fa5414772d5c445dac3c@syzkaller.appspotmail.com>
Reported-by: Hyunwoo Kim <v4bel@theori.io>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>