tty: split the lock up a bit further

The tty count sanity check may need the BKL, that isn't clear. However it
is clear that the count use of the lock is internal and independant of the
bigger use of the lock.

Furthermore the file list locking is also separately locked already
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/char/tty_io.c

@@ -516,6 +516,8 @@ static void do_tty_hangup(struct work_struct *work)
 	/* 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 ? */
 	list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
@@ -529,6 +531,7 @@ static void do_tty_hangup(struct work_struct *work)
 	}
 	file_list_unlock();
 
+	lock_kernel();
 	tty_ldisc_hangup(tty);
 
 	read_lock(&tasklist_lock);