scsi: st: New session only when Unit Attention for new tape

Currently the code starts new tape session when any Unit Attention
(UA) is seen when opening the device. This leads to incorrectly
clearing pos_unknown when the UA is for reset. Set new session only
when the UA is for a new tape.

Signed-off-by: Kai Mäkisara <Kai.Makisara@kolumbus.fi>
Link: https://lore.kernel.org/r/20241106095723.63254-4-Kai.Makisara@kolumbus.fi


Reviewed-by: John Meneghini <jmeneghi@redhat.com>
Tested-by: John Meneghini <jmeneghi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>