ata_xpt: Rename probe_softc to aprobe_softc

Since both scsi_xpt and ata_xpt use the same name for the softc, this
can lead to problems in gdb. Avoid the issue by renaming the ata
probe_softc to aprobe_softc as has been done for the aprobe in
0f280cbd0a3a9. This was overlooked at the time.

Sponsored by: Netflix
MFC After: 2 weeks

(cherry picked from commit 4762aa57ad17b67b46570025ca21ac4a5eefed0c)