Bluetooth: hci_sock: Forward credentials to monitor

This stores scm_creds into hci_skb_cb so they can be properly forwarded
to the likes of btmon which is then able to print information about the
process who is originating the traffic:

bluetoothd[35]: @ MGMT Command: Rea.. (0x0001) plen 0  {0x0001}
@ MGMT Event: Command Complete (0x0001) plen 6         {0x0001}
      Read Management Version Information (0x0001) plen 3

bluetoothd[35]: < ACL Data T.. flags 0x00 dlen 41
      ATT: Write Command (0x52) len 36
        Handle: 0x0043 Type: ASE Control Point (0x2bc6)
          Data: 020203000110270000022800020a00409c0001000110270000022800020a00409c00
            Opcode: QoS Configuration (0x02)
            Number of ASE(s): 2
            ASE: #0
            ASE ID: 0x03
            CIG ID: 0x00
            CIS ID: 0x01
            SDU Interval: 10000 usec
            Framing: Unframed (0x00)
            PHY: 0x02
            LE 2M PHY (0x02)
            Max SDU: 40
            RTN: 2
            Max Transport Latency: 10
            Presentation Delay: 40000 us
            ASE: #1
            ASE ID: 0x01
            CIG ID: 0x00
            CIS ID: 0x01
            SDU Interval: 10000 usec
            Framing: Unframed (0x00)
            PHY: 0x02
            LE 2M PHY (0x02)
            Max SDU: 40
            RTN: 2
            Max Transport Latency: 10
            Presentation Delay: 40000 us

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>