stand: Use calloc instead of malloc for initialization of filesystem devsw

[audrey_rhelmot.io]

Summary

This change is required for https://reviews.freebsd.org/D49355, so that we can check if d_dev is uninitialized by checking if it's NULL.

[imp]

Nice refactor! Thanks!

[imp]

Oh! I should have committed this ages ago.
Still need to look more closely at the label patch. Love the concept, unsure about a couple of details.

[pfg]

Hmm ...
Sorry to be picky here .. but if you look at the calloc(3) manpage:
void *calloc(size_t number, size_t size);

The number comes first, then the size, so those should all have been calloc(1, sizeof(struct XXX))