gfs2: write revokes should traverse sd_ail1_list in reverse

All the other functions that deal with the sd_ail_list run the list
from the tail back to the head, iow, in reverse. We should do the
same while writing revokes, otherwise we might miss removing entries
properly from the list when we hit the limit of how many revokes we
can write at one time (based on block size, which determines how
many block pointers will fit in the revoke block).

Signed-off-by: Bob Peterson <rpeterso@redhat.com>