btrfs: open code btrfs_bio_free_csum

btrfs_bio_free_csum has only one caller left, and that caller is always
for an data inode and doesn't need zeroing of the csum pointer as that
pointer will never be touched again.  Just open code the conditional
kfree there.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>