dm snapshot: use GFP_KERNEL when initializing exceptions

The list of initial exceptions is loaded in the target constructor.  We
are allowed to allocate memory with GFP_KERNEL at this point.  So,
change alloc_completed_exception to use GFP_KERNEL when being called
from the constructor.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>