tty: Fix BKL taken under a spinlock bug introduced in the BKL split

The fasync path takes the BKL (it probably doesn't need to in fact)
while holding the file_list spinlock. You can't do that with the kernel
lock: it causes lock inversions and deadlocks.

Leave the BKL over that bit for the moment.

Identified by AKPM.

Signed-off-by: Alan Cox <[e-mail]>
Acked-and-Tested-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_io.c

···
 	/* inuse_filps is protected by the single kernel lock */
 	lock_kernel();
 	check_tty_count(tty, "do_tty_hangup");
-	unlock_kernel();
 
 	file_list_lock();
 	/* This breaks for file handles being sent over AF_UNIX sockets ? */
···
 	}
 	file_list_unlock();
 
-	lock_kernel();
 	tty_ldisc_hangup(tty);
 
 	read_lock(&tasklist_lock);