watchdog: update watchdog attributes atomically

proc_dowatchdog doesn't synchronize multiple callers which might lead to
confusion when two parallel callers might confuse watchdog_enable_all_cpus
resp watchdog_disable_all_cpus (eg watchdog gets enabled even if
watchdog_thresh was set to 0 already).

This patch adds a local mutex which synchronizes callers to the sysctl
handler.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>