Btrfs: fix the comment typo for btrfs_attach_transaction_barrier

The comment is for btrfs_attach_transaction_barrier, not for
btrfs_attach_transaction. Fix the typo.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Acked-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>