This is a fix for when we encounter an alias during insert. When that
happens we move the request to dispatch, but it may happen that this
request is also the ->last_merge hint. So we may attempt to merge with
this later, when it's either in progress or already freed. Rearrange the
logic a bit so we clear the merge hint there as well. It looks more
complex than it is, the only real code change is the addition of a
cfq_remove_merge_hints() in cfq_dispatch_sort().
Signed-off-by: Linus Torvalds <torvalds@osdl.org>