Staging: lustre: ptlrpc: signedness bug in high_priority_ratio_store()

We want to store a non-negative int here.  The original code had a check
for unsigned long less than zero which is a mistake but also casting
from a positive long to an int can result in a negative number.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>