Bluetooth: MGMT: validate advertising TLV before type checks

Bluetooth: MGMT: validate advertising TLV before type checks

tlv_data_is_valid() reads each advertising data field length from
data[i], then inspects data[i + 1] for managed EIR types before
checking that the current field still fits inside the supplied buffer.

A malformed field whose length byte is the last byte of the buffer can
therefore make the parser read one byte past the advertising data.

KASAN reported the following when a malformed MGMT_OP_ADD_ADVERTISING
request reached that path:

  BUG: KASAN: vmalloc-out-of-bounds in tlv_data_is_valid()
  Read of size 1
  Call trace:
    tlv_data_is_valid()
    add_advertising()
    hci_mgmt_cmd()
    hci_sock_sendmsg()

Move the existing element-length check before any type-octet inspection
so each non-empty element is proven to contain its type byte before the
parser looks at data[i + 1].

Fixes: 2bb36870e8cb ("Bluetooth: Unify advertising instance flags check")
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>