[PATCH] dquot: add proper locking when using current->signal->tty

Dquot passes the tty to tty_write_message without locking

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>