vxlan: fix error return code in vxlan_fdb_append

[ Upstream commit 7cea5560bf656b84f9ed01c0cc829d4eecd0640b ]

When kmalloc and dst_cache_init failed,
should return ENOMEM rather than ENOBUFS.

Signed-off-by: Hongbin Wang <wh_bin@126.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>