net: vxlan: make vxlan_remcsum() return drop reasons

Make vxlan_remcsum() support skb drop reasons by changing the return
value type of it from bool to enum skb_drop_reason.

The only drop reason in vxlan_remcsum() comes from pskb_may_pull_reason(),
so we just return it.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>