xfs: Fix comment on xfs_trans_ail_update_bulk()

This function doesn't take the AIL lock, but should be called
with AIL lock held. Also (hopefuly) simplify the comment.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanbabu@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>