gfs2: make gfs2_ail1_empty_one return the count of active items

This patch is one baby step toward simplifying the journal management.
It simply changes function gfs2_ail1_empty_one from a void to an int and
makes it return a count of active items. This allows the caller to check
the return code rather than list_empty on the tr_ail1_list. This way
we can, in a later patch, combine transaction ail1 and ail2 lists.

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