devlink: Add missing genlmsg_cancel() in devlink_nl_sb_port_pool_fill()

If sb_occ_port_pool_get() failed in devlink_nl_sb_port_pool_fill(),
msg should be canceled by genlmsg_cancel().

Fixes: df38dafd

 ("devlink: implement shared buffer occupancy monitoring interface")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Link: https://lore.kernel.org/r/20201113111622.11040-1-wanghai38@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>