release_mem contains two copies of exactly the same code.  Refactor these
into a new helper, release_tty.  The only change in behaviour is that the
driver reference count is now decremented after the master tty has been
freed instead of before.

[penberg@cs.helsinki.fi: fix use-after-free in release_tty.]
Cc: Alan Cox <alan@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>