Eliminate the open_mutex after complaints from Blaisorblade.  It turns out
that the tty count provides the information needed to tell whether we are the
first opener or last closer.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>