Corrections in Documentation/block/ioprio.txt

The newer glibc does not allow system calls to be made via _syscallN()
wrapper. They have to be made through syscall(). The ionice code used
the older interface. Correcting it to use syscall.

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>