s390/dumpstack: use pr_cont where appropriate

Use pr_cont instead of simple printk calls when lines will be
continued. This fixes the kernel output of various lines printed on
e.g. a warning:

Before:
[    0.840604] Krnl PSW : 0404c00180000000 000000000017d1d2
[    0.840606]  (try_to_wake_up+0x382/0x5e0)

[    0.840610]            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0
[    0.840611]  RI:0 EA:3

After:
[    0.831772] Krnl PSW : 0404c00180000000 000000000017d14a (try_to_wake_up+0x382/0x5e0)
[    0.831776]            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3

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