lightnvm: check overflow and correct mlc pairs

The specification currently limits the number of MLC pairs to 886. Make
sure that a device is unable to be instantiate if more is configured.

Also, previously the patch had the wrong math for copying MLC pairs, as
it only copied half of the actual entries.

Fixes: ca5927e7 "lightnvm: introduce mlc lower page table mappings"
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>