xfrm: provide message size for XFRM_MSG_MAPPING

xfrm: provide message size for XFRM_MSG_MAPPING

The compat 64=>32 translation path handles XFRM_MSG_MAPPING, but
xfrm_msg_min[] does not provide the native payload size for this
message type.

Add the missing XFRM_MSG_MAPPING entry so compat translation can size
and translate mapping notifications correctly.

Fixes: 5461fc0c8d9f ("xfrm/compat: Add 64=>32-bit messages translator")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Ruijie Li <ruijieli51@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>