Hold the reference count until the CCB is released

When a disk disappears and the periph is invalidated, any I/Os that
are pending with the controller can cause a crash when they
complete. Move to holding the softc reference count taken in dastart()
until the I/O is complete rather than only until xpt_action()
returns. (This approach was suggested by Ken Merry.)

Sponsored by: Netflix
Submitted by: Chuck Silvers
Differential Revision: https://reviews.freebsd.org/D15435