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.

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