tty: kgdb_nmi: Automatically manage tty enable

At present it is not possible to boot with the ttyNMI0 console treating
character input normally, instead character input triggers a prompt
telling the user how to trigger the knock detector and enter the
debugger. To use the console normally requires that kdb be entered and
the nmi_console command be used to enable the console (or if only kgdb
is present then gdb must directly manipulate the value of
kgdb_nmi_tty_enabled).

This patch automates the management of kgdb_nmi_tty_enabled by keeping
track of the number of file handles that are open for reading and using
that to determine how to tty should operate.

The approach means that:

1. Behaviour before init starts is unchanged.

2. If the userspace runs a getty or some other interactive process on
   /dev/console (or explicitly on /dev/ttyNMI0) the tty will treat
   character input like any other tty.

3. If the userspace doesn't use /dev/console or if it uses /dev/console
   only to log messages (O_WRONLY) then the user prompt is retained.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-serial@vger.kernel.org
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: kgdb-bugreport@lists.sourceforge.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>