When accessing the mc_devices list of memory controller descriptors, we
need to hold mem_ctls_mutex. This was not always the case, fix that.

Make all external callers call a version which grabs the mutex since the
last is local to edac_mc.c.
Reported-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>