virtio-blk: drop config_mutex

virtio-blk: drop config_mutex

config_mutex served two purposes: prevent multiple concurrent config
change handlers, and synchronize access to config_enable flag.

Since commit 51b2a627a90beb6efbb66ebf4d9a68c99dd43156
    workqueue: make all workqueues non-reentrant
all workqueues are non-reentrant, and config_enable
is now gone.

Get rid of the unnecessary lock.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>