scsi sg: remove incorrect scsi command checking logic

The SCSI_IOCTL_SEND_COMMAND ioctl has interesting scsi command
"security" checking.

If the file was opened read-only (but only in that case), it will
fetch the first byte of the command from user space, and do
"sg_allow_access()" on it.  That, in turn, will check that
"blk_verify_command()" is ok with that command byte.

If that passes, it will then do call "sg_scsi_ioctl()" to execute
the command.

This is entirely nonsensical for several reasons.

It's nonsensical simply because it's racy: after it copies the command
byte from user mode to check it, user mode could just change the byte
before it is actually submitted later by "sg_scsi_ioctl()".

But it is nonsensical also because "sg_scsi_ioctl()" itself already does
blk_verify_command() on the command properly after it has been copied
from user space.

So it is an incorrect implementation of a pointless check. Remove it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>