kernel/watchdog.c: control hard lockup detection default

In some cases we don't want hard lockup detection enabled by default.
An example is when running as a guest.  Introduce

  watchdog_enable_hardlockup_detector(bool)

allowing those cases to disable hard lockup detection.  This must be
executed early by the boot processor from e.g.  smp_prepare_boot_cpu, in
order to allow kernel command line arguments to override it, as well as
to avoid hard lockup detection being enabled before we've had a chance
to indicate that it's unwanted.  In summary,

  initial boot:					default=enabled
  smp_prepare_boot_cpu
    watchdog_enable_hardlockup_detector(false):	default=disabled
  cmdline has 'nmi_watchdog=1':			default=enabled

The running kernel still has the ability to enable/disable at any time
with /proc/sys/kernel/nmi_watchdog us usual.  However even when the
default has been overridden /proc/sys/kernel/nmi_watchdog will initially
show '1'.  To truly turn it on one must disable/enable it, i.e.

  echo 0 > /proc/sys/kernel/nmi_watchdog
  echo 1 > /proc/sys/kernel/nmi_watchdog

This patch will be immediately useful for KVM with the next patch of this
series.  Other hypervisor guest types may find it useful as well.

[akpm@linux-foundation.org: fix build]
[dzickus@redhat.com: fix compile issues on sparc]
Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>