af_key: fix leaks in key_pol_get_resp and dump_sp.

In both functions, if pfkey_xfrm_policy2msg failed we leaked the newly
allocated sk_buff.  Free it on error.

Fixes: 55569ce2

 ("Fix conversion between IPSEC_MODE_xxx and XFRM_MODE_xxx.")
Reported-by:  <syzbot+4f0529365f7f2208d9f0@syzkaller.appspotmail.com>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>