dm crypt: don't decrease device limits

dm-crypt should only increase device limits, it should not decrease them.

This fixes a bug where the user could creates a crypt device with 1024
sector size on the top of scsi device that had 4096 logical block size.
The limit 4096 would be lost and the user could incorrectly send
1024-I/Os to the crypt device.

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