scsi: 3w-xxxx: fix a missing-check bug

In tw_chrdev_ioctl(), the length of the data buffer is firstly copied
from the userspace pointer 'argp' and saved to the kernel object
'data_buffer_length'. Then a security check is performed on it to make
sure that the length is not more than 'TW_MAX_IOCTL_SECTORS *
512'. Otherwise, an error code -EINVAL is returned. If the security
check is passed, the entire ioctl command is copied again from the
'argp' pointer and saved to the kernel object 'tw_ioctl'. Then, various
operations are performed on 'tw_ioctl' according to the 'cmd'. Given
that the 'argp' pointer resides in userspace, a malicious userspace
process can race to change the buffer length between the two
copies. This way, the user can bypass the security check and inject
invalid data buffer length. This can cause potential security issues in
the following execution.

This patch checks for capable(CAP_SYS_ADMIN) in tw_chrdev_open() to
avoid the above issues.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>