btrfs: replace BUG_ON() at split_item() with proper error handling

There's no need to BUG_ON() at split_item() if the leaf does not have
enough free space for the new item, we can just return -ENOSPC since
the caller can deal with errors from split_item(). Also, as this is a
very unlikely condition to happen, because the caller has invoked
setup_leaf_for_split() before calling split_item(), surround the
condition with a WARN_ON() which makes it easier to notice this
unexpected condition and tags the if branch with 'unlikely' as well.

I've actually once hit this BUG_ON() with some incorrect code changes
I had, which was very inconvenient as it required rebooting the VM.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -4000,7 +4000,12 @@ static noinline int split_item(struct btrfs_path *path,
 	struct btrfs_disk_key disk_key;
 
 	leaf = path->nodes[0];
-	BUG_ON(btrfs_leaf_free_space(leaf) < sizeof(struct btrfs_item));
+	/*
+	 * Shouldn't happen because the caller must have previously called
+	 * setup_leaf_for_split() to make room for the new item in the leaf.
+	 */
+	if (WARN_ON(btrfs_leaf_free_space(leaf) < sizeof(struct btrfs_item)))
+		return -ENOSPC;
 
 	orig_slot = path->slots[0];
 	orig_offset = btrfs_item_offset(leaf, path->slots[0]);