kernel/nataraid: Fix a panic upon booting with a degraded Intel RAID.

disk_idx has upper bits set in this case which we need to mask.

Taken-from:             FreeBSD's r205074
Reported-and-tested-by: Aaron Bieber <deftly@gmail.com>
libcalendar: Use our own definition of NULL, instead of rolling an own.
kernel/nataraid: Fix an issue with the devstat support I added recently.

This fixes the "devstat_end_transaction: HELP!! busy_count for ar0 is < 0"
we were seeing.

Tested-by: Aaron Bieber <deftly@gmail.com>