btrfs: remove unneeded NULL checks before kfree

Kfree has taken the NULL pointer into account. So remove the check
before kfree.

The issue is detected with the help of Coccinelle.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>