s390/ap: Rename mutex ap_perms_mutex to ap_attr_mutex

s390/ap: Rename mutex ap_perms_mutex to ap_attr_mutex

The mutex ap_perms_mutex was already used not only for protection
of the struct ap_perms ap_perms variable but also for an consistent
update of the AP bus sysfs attributes apmask and aqmask.

So rename this mutex to ap_attr_mutex which better reflects the
current use. This is also a preparation for an upcoming patch which
will use this mutex to lock updates on a new sysfs attribute.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>