erofs: fix deadlock when shrink erofs slab

We observed the following deadlock in the stress test under low
memory scenario:

Thread A Thread B
- erofs_shrink_scan
- erofs_try_to_release_workgroup
- erofs_workgroup_try_to_freeze -- A
- z_erofs_do_read_page
- z_erofs_collection_begin
- z_erofs_register_collection
- erofs_insert_workgroup
- xa_lock(&sbi->managed_pslots) -- B
- erofs_workgroup_get
- erofs_wait_on_workgroup_freezed -- A
- xa_erase
- xa_lock(&sbi->managed_pslots) -- B

To fix this, it needs to hold xa_lock before freezing the workgroup
since xarray will be touched then. So let's hold the lock before
accessing each workgroup, just like what we did with the radix tree
before.

[ Gao Xiang: Jianhua Hao also reports this issue at
https://lore.kernel.org/r/[e-mail] ]

Link: https://lore.kernel.org/r/[e-mail]
Fixes: 64094a04414f ("erofs: convert workstn to XArray")
Reviewed-by: Chao Yu <[e-mail]>
Reviewed-by: Gao Xiang <[e-mail]>
Signed-off-by: Huang Jianan <[e-mail]>
Reported-by: Jianhua Hao <[e-mail]>
Signed-off-by: Gao Xiang <[e-mail]>

fs/erofs/utils.c

···
 	 * however in order to avoid some race conditions, add a
 	 * DBG_BUGON to observe this in advance.
 	 */
-	DBG_BUGON(xa_erase(&sbi->managed_pslots, grp->index) != grp);
+	DBG_BUGON(__xa_erase(&sbi->managed_pslots, grp->index) != grp);
 
 	/* last refcount should be connected with its managed pslot.  */
 	erofs_workgroup_unfreeze(grp, 0);
···
 	unsigned int freed = 0;
 	unsigned long index;
 
+	xa_lock(&sbi->managed_pslots);
 	xa_for_each(&sbi->managed_pslots, index, grp) {
 		/* try to shrink each valid workgroup */
 		if (!erofs_try_to_release_workgroup(sbi, grp))
 			continue;
+		xa_unlock(&sbi->managed_pslots);
 
 		++freed;
 		if (!--nr_shrink)
-			break;
+			return freed;
+		xa_lock(&sbi->managed_pslots);
 	}
+	xa_unlock(&sbi->managed_pslots);
 	return freed;
 }