btrfs: fix adding block group to a reclaim list and the unused list during reclaim

There is a potential parallel list adding for retrying in
btrfs_reclaim_bgs_work and adding to the unused list. Since the block
group is removed from the reclaim list and it is on a relocation work,
it can be added into the unused list in parallel. When that happens,
adding it to the reclaim list will corrupt the list head and trigger
list corruption like below.

Fix it by taking fs_info->unused_bgs_lock.

  [177.504][T2585409] BTRFS error (device nullb1): error relocating ch= unk 2415919104
  [177.514][T2585409] list_del corruption. next->prev should be ff1100= 0344b119c0, but was ff11000377e87c70. (next=3Dff110002390cd9c0)
  [177.529][T2585409] ------------[ cut here ]------------
  [177.537][T2585409] kernel BUG at lib/list_debug.c:65!
  [177.545][T2585409] Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
  [177.555][T2585409] CPU: 9 PID: 2585409 Comm: kworker/u128:2 Tainted: G        W          6.10.0-rc5-kts #1
...