x86/mcelog: Get rid of RCU remnants

Jeremy reported a suspicious RCU usage warning in mcelog.

/dev/mcelog is called in process context now as part of the notifier
chain and doesn't need any of the fancy RCU and lockless accesses which
it did in atomic context.

Axe it all in favor of a simple mutex synchronization which cures the
problem reported.

Fixes: 5de97c9f

 ("x86/mce: Factor out and deprecate the /dev/mcelog driver")
Reported-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-and-tested-by: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: linux-edac@vger.kernel.org
Cc: Laura Abbott <labbott@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20171101164754.xzzmskl4ngrqc5br@pd.tnic
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1498969