scsi: mptfusion: Fix double fetch bug in ioctl

Tom Hatskevich reported that we look up "iocp" then, in the called
functions we do a second copy_from_user() and look it up again.
The problem that could cause is:

drivers/message/fusion/mptctl.c
   674          /* All of these commands require an interrupt or
   675           * are unknown/illegal.
   676           */
   677          if ((ret = mptctl_syscall_down(iocp, nonblock)) != 0)
                                               ^^^^
We take this lock.

   678                  return ret;
   679
   680          if (cmd == MPTFWDOWNLOAD)
   681                  ret = mptctl_fw_download(arg);
                                                 ^^^
Then the user memory changes and we look up "iocp" again but a different
one so now we are holding the incorrect lock and have a race condition.

   682          else if (cmd == MPTCOMMAND)
   683                  ret = mptctl_mpt_command(arg);

The security impact of this bug is not as bad as it could have been
be...