All callers pass the eb::level so we can get read it directly inside the
btrfs_bin_search and key_search.

This is inspired by the work of Marek in U-boot.

CC: Marek Behun <marek.behun@nic.cz>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>