libcam(3): 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.

Reviewed By: imp, rpokala (manpages)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27333