MFC some nullfs code

[Dmitry Sivachenko]

Hello!

Could someone please MFC 1.50-1.51 delta of null_vnops.c (committed by
mkcusick about a year ago).  It would not require much work.

It fixes panic in the following test:
compile nullfs statically with options DIAGNOSTIC;
# mount_null /bin /mnt
# file /mnt/ps  // any open()/close() on mounted filesystem
# umount /mnt

System will panic.
I tested this patch with several buldworlds on nullfs-mounted
/usr/src, and it seems to work fine.

Thanks!