dm stats: limit the number of entries

The kvmalloc function fails with a warning if the size is larger than
INT_MAX. Linus said that there should be limits that prevent this warning
from being hit. This commit adds the limits to the dm-stats subsystem
in DM core.

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