btrfs: use RCU for quick device check in btrfs_init_new_device

When adding a new device there's a mandatory check to see if a device is
being duplicated to the filesystem it's added to. Since this is a
read-only operations not necessary to take device_list_mutex and can simply
make do with an rcu-readlock.

Using just RCU is safe because there won't be another device add delete
running in parallel as btrfs_init_new_device is called only from
btrfs_ioctl_add_dev.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>