[SCSI] sr: partial revert of 24669f75

The patch

[SCSI] SCSI core kmalloc2kzalloc

Has an incorrect piece in sr_ioctl.c; it changes buffer from kmalloc
to kzalloc, but then removes the clearing of the stack variable struct
packet_command.  This, in turn leaves rubbish in the sense pointer
which the sr_do_ioctl() command then happily writes to ... oops.

Thanks to Mike Christie <michaelc@cs.wisc.edu> for spotting this.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>