srch compaction needs to prepare alloc for commit

The srch client compaction work initializes allocators, dirties blocks,
and writes them out as its transaction.  It forgot to call the
pre-commit allocator prepare function.

The prepare function drops block references used by the meta allocator
during the transaction.  This leaked block references which kept blocks
from being freed by the shrinker under memory pressure.  Eventually
memory was full of leaked blocks and the shrinker walked all of them
looking blocks to free, resulting in an effective livelock that ground
the system to a crawl.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/block.c

@@ -396,6 +396,7 @@ static void block_remove_all(struct super_block *sb)
 
 		if (block_get_if_inserted(bp)) {
 			block_remove(sb, bp);
+			WARN_ON_ONCE(atomic_read(&bp->refcount) != 1);
 			block_put(sb, bp);
 		}
 	}

kmod/src/srch.c

@@ -2156,7 +2156,8 @@ static void scoutfs_srch_compact_worker(struct work_struct *work)
 	if (ret < 0)
 		goto commit;
 
-	ret = scoutfs_block_writer_write(sb, &wri);
+	ret = scoutfs_alloc_prepare_commit(sb, &alloc, &wri) ?:
+	      scoutfs_block_writer_write(sb, &wri);
 commit:
 	/* the server won't use our partial compact if _ERROR is set */
 	sc->meta_avail = alloc.avail;