btrfs: zoned: fix lock ordering in btrfs_zone_activate()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: zoned: fix lock ordering in btrfs_zone_activate()

The btrfs CI reported a lockdep warning as follows by running generic
generic/129.

WARNING: possible circular locking dependency detected
6.7.0-rc5+ #1 Not tainted
------------------------------------------------------
kworker/u5:5/793427 is trying to acquire lock:
ffff88813256d028 (&cache->lock){+.+.}-{2:2}, at: btrfs_zone_finish_one_bg+0x5e/0x130
but task is already holding lock:
ffff88810a23a318 (&fs_info->zone_active_bgs_lock){+.+.}-{2:2}, at: btrfs_zone_finish_one_bg+0x34/0x130
which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:
-> #1 (&fs_info->zone_active_bgs_lock){+.+.}-{2:2}:
...
-> #0 (&cache->lock){+.+.}-{2:2}:
...

This is because we take fs_info->zone_active_bgs_lock after a block_group's
lock in btrfs_zone_activate() while doing the opposite in other places.

Fix the issue by expanding the fs_info->zone_active_bgs_lock's critical
section and taking it before a block_group's lock.