powerpc/xmon: fix task state output

p_state is unsigned since the commit 2f064a59

The patch also uses TASK_RUNNING instead of null.

Fixes: 2f064a59 ("sched: Change task_struct::state")
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211026133108.7113-1-kda@linux-powerpc.org