wireless: Add missing locking to cfg80211_dev_rename

device_rename only performs useful and race free validity
checking at the optional sysfs level so depending on it
for all of the validity checking in cfg80211_dev_rename
is racy.

Instead implement all of the needed validity checking
and locking in cfg80211_dev_rename.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>