ionice: fix -p

ionice: fix -p

 $ ionice 123
 none: prio 4
 none: prio 4

It calls ioprio_get(0x1, 0) and ioprio_get(0x1, 123), because the
code does not check it the "-p" options was specified.

The proper command line syntax is:

  $ ionice -p 123

and the ioprio_get() should be called only once.

Signed-off-by: Karel Zak <kzak@redhat.com>