s390/dumpstack: print psw mask and address again

With pointer obfuscation the output of show_registers() became quite useless:

Krnl PSW : (____ptrval____) (____ptrval____) (__list_add_valid+0x98/0xa8)

In order to print the psw mask and address use %px instead of %p.
And the output looks again like this:

Krnl PSW : 0404d00180000000 00000000007c0dd0 (__list_add_valid+0x98/0xa8)

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>