net: af_key: add check for pfkey_broadcast in function pfkey_process

[ Upstream commit 4dc2a5a8f6754492180741facf2a8787f2c415d7 ]

If skb_clone() returns null pointer, pfkey_broadcast() will
return error.
Therefore, it should be better to check the return value of
pfkey_broadcast() and return error if fails.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>