tty: Fix unbalanced BKL handling in error path

Arnd noted:

After the "retry_open:" label, we first get the tty_mutex
and then the BKL. However a the end of tty_open, we jump
back to retry_open with the BKL still held. If we run into
this case, the tty_open function will be left with the BKL
still held.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>