Backport d73e17281665 to v4.15 stable

[Lukas Wunner]

Dear Greg,

when you refill the patch queue for v4.15 stable one of these days,
please consider adding

    commit d73e172816652772114827abaa2dbc053eecbbd7
    Author: Lukas Wunner <lukas@xxxxxxxxx>
    Date:   Fri Nov 17 00:54:53 2017 +0100
    
    Bluetooth: hci_serdev: Init hci_uart proto_lock to avoid oops

to fix a boot crash on certain machines with UART-attached Bluetooth
which was kindly reported by John Stultz.  The patch unfortunately
lacked a Fixes: or Cc: stable tag.  It landed in Linus' tree 7 hours ago.

Thanks!

Lukas