Bluetooth: MGMT: reject HCI_CMD_SYNC params_len above 255

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: MGMT: reject HCI_CMD_SYNC params_len above 255

mgmt_hci_cmd_sync() checks that the message length agrees with params_len
but puts no upper bound on it. params_len is __le16 while the parameter
length in the HCI command header is a u8:

struct hci_command_hdr {
__le16 opcode;
__u8 plen;
} __packed;

hci_cmd_sync_alloc() assigns one to the other:

hdr->plen = plen;

if (plen)
skb_put_data(skb, param, plen);

so a params_len of 256 leaves plen at 0 while all 256 bytes are still
appended. The frame handed to the driver then declares no parameters and
carries 256 of them. On a length framed transport such as H:4 the
controller takes the trailing bytes as the start of the next packet.

The mgmt socket MTU is HCI_MAX_FRAME_SIZE, so params_len can reach about
1KB this way. Commit 03f1700b9b4d ("Bluetooth: MGMT: reject malformed
HCI_CMD_SYNC commands") only made params_len agree with the message
length, a value that fits the message but not the header field is still
accepted.

Reject params_len that does not fit the header field.