gfs2: fix use-after-free on transaction ail lists

Before this patch, transactions could be merged into the system
transaction by function gfs2_merge_trans(), but the transaction ail
lists were never merged. Because the ail flushing mechanism can run
separately, bd elements can be attached to the transaction's buffer
list during the transaction (trans_add_meta, etc) but quickly moved
to its ail lists. Later, in function gfs2_trans_end, the transaction
can be freed (by gfs2_trans_end) while it still has bd elements
queued to its ail lists, which can cause it to either lose track of
the bd elements altogether (memory leak) or worse, reference the bd
elements after the parent transaction has been freed.

Although I've not seen any serious consequences, the problem becomes
apparent with the previous patch's addition of:

	gfs2_assert_warn(sdp, list_empty(&tr->tr_ail1_list));

to function gfs2_trans_free().

This patch adds logic into gfs2_merge_trans() to move the merged
transaction's ail lists to the sdp transaction. This prevents the
use-after-free. To do this properly, we need to hold the ail lock,
so we pass sdp into the function instead of the transaction itself.

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