tty: implement BTM as mutex instead of BKL

The tty locking now follows the rules for mutexes, so
we can replace the BKL usage with a new subsystem
wide mutex.

Using a regular mutex here will change the behaviour
when blocked on the BTM from spinning to sleeping,
but that should not be visible to the user.

Using the mutex also means that all the BTM is now
covered by lockdep.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>