Be consistent with M_ZERO when allocating ccbs.

Be consistent with M_ZERO when allocating ccbs.

There are four places, all in cam_xpt.c, where ccbs are malloc'ed. Two of
these use M_ZERO, two don't. The two that don't meant that allocated ccbs
had trash in them making it hard to debug errors where they showed up. Due
to this, use M_ZERO all the time when allocating ccbs.

Submitted by:	Scott Ferris <scott.ferris@isilon.com>
Sponsored by:	EMC/Isilon Storage Division
Reviewed by:	scottl, imp
Differential:	https://reviews.freebsd.org/D2120