mtd: mtdblock: remove the needless mtdblks_lock

The global lock mtdblks_lock was used to protect the original mtdblks
array to avoid race conditions. As the mtdblks array was already gone,
but the mtdblks_lock is left, and it causes latency when open/release dev.
So we need to remove it here.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>