btrfs: do not BUG_ON() on tree mod log failure at __btrfs_cow_block()

At __btrfs_cow_block(), instead of doing a BUG_ON() in case we fail to
record a tree mod log root insertion operation, do a transaction abort
instead. There's really no need for the BUG_ON(), we can properly
release all resources in this context and turn the filesystem to RO mode
and in an error state instead.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ctree.c

@@ -584,9 +584,14 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans,
 		    btrfs_header_backref_rev(buf) < BTRFS_MIXED_BACKREF_REV)
 			parent_start = buf->start;
 
-		atomic_inc(&cow->refs);
 		ret = btrfs_tree_mod_log_insert_root(root->node, cow, true);
-		BUG_ON(ret < 0);
+		if (ret < 0) {
+			btrfs_tree_unlock(cow);
+			free_extent_buffer(cow);
+			btrfs_abort_transaction(trans, ret);
+			return ret;
+		}
+		atomic_inc(&cow->refs);
 		rcu_assign_pointer(root->node, cow);
 
 		btrfs_free_tree_block(trans, btrfs_root_id(root), buf,