Initialize async_arg_ptr in xpt_async when called with async_code
AC_ADVINFO_CHANGED.

Without this change, newly inserted hard disks won't always have their
physical path device nodes created. The problem reproduces most readily
when attaching a large number of disks at once.

Differential Revision: https://reviews.freebsd.org/D2290
Reviewed by: mav, imp
MFC after: 2 weeks
Sponsored by: Spectra Logic