edac_core: Don't let free(mci) happen while using it

edac_core: Don't let free(mci) happen while using it

A very nasty bug were happening on edac core, due to the way mci objects are
freed. mci memory is freed when kobject count reaches zero, by
edac_mci_control_release(). However, from the logs, this is clearly happening
before the final usage of mci struct:

[15799.607454] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 640: edac_mci_control_release() mci instance idx=0 releasing
[15799.618773] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 769: edac_inst_grp_release()
[15799.627326] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 894: edac_remove_mci_instance_attributes() end of seeking for group all_channel_counts
[15799.640887] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 877: edac_remove_mci_instance_attributes() sysfs_attrib = ffffffffa01d7240
[15799.653412] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 1020: edac_remove_sysfs_mci_device()  remove_link
[15799.663753] EDAC DEBUG: in drivers/edac/edac_mc_sysfs.c, line at 1024: edac_remove_sysfs_mci_device()  remove_mci_instance

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>