[PATCH] do_acct_process(): don't take tty_mutex

No need to take the global tty_mutex, signal->tty->driver can't go away while
we are holding ->siglock.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>