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>