dm array: fix bug in growing array

commit 9c1d4de56066e4d6abc66ec188faafd7b303fb08 upstream.

Entries would be lost if the old tail block was partially filled.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>