[SCSI] st: show options currently set in sysfs

Show the current binary tape driver and mode options is sysfs. A file
(options) is created in each directory in /sys/class/scsi_tape. The files
contain masks showing the options. The mask bit definitions are the same as
used when setting the options using the MTSETDRVBUFFER function in the
MTIOCTOP ioctl (defined in include/linux/mtio.h). For example:
> cat /sys/class/scsi_tape/nst0/options
0x00000d07

[jejb: updated doc with correction from Randy Dunlap]

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>