Fix a few issues related to the XPT_GDEV_ADVINFO CCB.
camcontrol.c: In buildbusdevlist(), don't attempt to get call
getdevid() for an unconfigured device, even when the verbose flag is set. The cam_open_btl() call will almost certainly fail. Probe for the buffer size when issuing the XPT_GDEV_ADVINFO CCB. Probing for the buffer size first helps us avoid allocating the maximum buffer size when it really may not be necessary. This also helps avoid errors from cam_periph_mapmem() if we attempt to map more than MAXPHYS.
cam_periph.c: In cam_periph_mapmem(), if the XPT_GDEV_ADVINFO CCB
shows a bufsiz of 0, we don't have anything to map, so just return. Also, set the maximum mapping size to MAXPHYS instead of DFLTPHYS for XPT_GDEV_ADVINFO CCBs, since they don't actually go down to the hardware.
scsi_pass.c: Don't bother mapping the buffer in XPT_GDEV_ADVINFO
CCBs if bufsiz is 0.