dm io: fix duplicate bio completion due to missing ref count

If only a subset of the devices associated with multiple regions support
a given special operation (eg. DISCARD) then the dec_count() that is
used to set error for the region must increment the io->count.

Otherwise, when the dec_count() is called it can cause the dm-io
caller's bio to be completed multiple times.  As was reported against
the dm-mirror target that had mirror legs with a mix of discard
capabilities.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=196077


Reported-by: Zhang Yi <yizhan@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>