Fix http://bugzilla.kernel.org/show_bug.cgi?id=6716

Doing a sysrq over a serial line into an SMP machine presently deadlocks.

Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Enrico Scholz <enrico.scholz@informatik.tu-chemnitz.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>