xfs: set cursor in xfs_ail_splice() even when AIL was empty

In xfs_ail_splice(), if a cursor is provided it is updated to
point to the last item on the list being spliced into the AIL.
But if the AIL was found to be empty, the cursor (if provided)
is just initialized instead.

There is no reason the empty AIL case needs to be treated any
differently.  And treating it the same way allows this code
to be rearranged a bit, with a somewhat tidier result.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>