dm cache: fix access beyond end of origin device

In order to avoid wasting cache space a partial block at the end of the
origin device is not cached.  Unfortunately, the check for such a
partial block at the end of the origin device was flawed.

Fix accesses beyond the end of the origin device that occured due to
attempted promotion of an undetected partial block by:

- initializing the per bio data struct to allow cache_end_io to work properly
- recognizing access to the partial block at the end of the origin device
- avoiding out of bounds access to the discard bitset

Otherwise, users can experience errors like the following:

 attempt to access beyond end of device
 dm-5: rw=0, want=20971520, limit=20971456
 ...
 device-mapper: cache: promotion failed; couldn't copy block

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org