dm cache: handle kmalloc failure allocating background_tracker struct

Currently there is no kmalloc failure check on the allocation of
the background_tracker struct in btracker_create(), and so a NULL return
will lead to a NULL pointer dereference.  Add a NULL check.

Detected by CoverityScan, CID#1416587 ("Dereference null return value")

Fixes: b29d4986 ("dm cache: significant rework to leverage dm-bio-prison-v2")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>