Make cam_getccb(3) zero the whole ccb, not just the header

[trasz]

Summary

Make cam_getccb(3) zero the whole ccb, not just the header.

Leaving zeroing to the clients leads to error-prone pointer
tricks (zeroing needs to preserve the CCB header), and this
code is not performance-critical, so there's really no reason
to not do it.

[rpokala]

LGTM, but I'm not authoritative when it comes to CAM.

[imp]

I believe this is a good change. There's no performance reason to avoid zeroing, and in the kernel we do this:

new_ccb = malloc(sizeof(*new_ccb), M_CAMCCB, M_ZERO|M_WAITOK);

so this makes code that runs in both places more similar.

[trasz]

(Tinderboxed.)