Btrfs: Remove unnecessary placeholder in btrfs_err_code

"notused" is not necessary. Set 1 to the first entry is enough.

Signed-off-by: Takeuchi Satoru <takeuchi_satoru@jp.fujitsu.com>
Cc: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>