MFC r280388 (by benno): Be consistent with M_ZERO when allocating ccbs.

MFC r280388 (by benno): 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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@288700 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f