Btrfs: Fix more false enospc errors and an oops from empty clustering

In comes cases the empty cluster was added twice to the total number of
bytes the allocator was trying to find.

With empty clustering on, the hint byte was sometimes outside of the
block group.  Add an extra goto to find the correct block group.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -2152,11 +2152,13 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 	}
 	search_start = max(search_start, first_logical_byte(root, 0));
 	search_start = max(search_start, hint_byte);
-	total_needed += empty_size;
 
-	if (search_start != last_wanted)
+	if (last_wanted && search_start != last_wanted) {
 		last_wanted = 0;
+		empty_size += empty_cluster;
+	}
 
+	total_needed += empty_size;
 	block_group = btrfs_lookup_block_group(root->fs_info, search_start);
 	if (!block_group)
 		block_group = btrfs_lookup_first_block_group(root->fs_info,
@@ -2171,7 +2173,9 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 		 * group thats not of the proper type, while looping this
 		 * should never happen
 		 */
-		WARN_ON(!block_group);
+		if (!block_group)
+			goto new_group_no_lock;
+
 		mutex_lock(&block_group->alloc_mutex);
 		if (unlikely(!block_group_bits(block_group, data)))
 			goto new_group;
@@ -2248,12 +2252,13 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 			break;
 		}
 new_group:
+		mutex_unlock(&block_group->alloc_mutex);
+new_group_no_lock:
 		last_wanted = 0;
-		if (loop > 0) {
+		if (!allowed_chunk_alloc && loop > 0) {
 			total_needed -= empty_cluster;
 			empty_cluster = 0;
 		}
-		mutex_unlock(&block_group->alloc_mutex);
 		/*
 		 * Here's how this works.
 		 * loop == 0: we were searching a block group via a hint
@@ -2271,6 +2276,10 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 			cur = head->next;
 			loop++;
 		} else if (loop == 1 && cur == head) {
+
+			total_needed -= empty_cluster;
+			empty_cluster = 0;
+
 			if (allowed_chunk_alloc && !chunk_alloc_done) {
 				up_read(&space_info->groups_sem);
 				ret = do_chunk_alloc(trans, root, num_bytes +