s390/hibernate: fix save and restore of kernel text section

Sebastian reported a crash caused by a jump label mismatch after resume.
This happens because we do not save the kernel text section during suspend
and therefore also do not restore it during resume, but use the kernel image
that restores the old system.

This means that after a suspend/resume cycle we lost all modifications done
to the kernel text section.
The reason for this is the pfn_is_nosave() function, which incorrectly
returns that read-only pages don't need to be saved. This is incorrect since
we mark the kernel text section read-only.
We still need to make sure to not save and restore pages contained within
NSS and DCSS segment.
To fix this add an extra case for the kernel text section and only save
those pages if they are not contained within an NSS segment.

Fixes the following crash (and the above bugs as well):

Jump label code mismatch at netif_receive_skb_internal+0x28/0xd0
Found:    c0 04 00 00 00 00
Expected: c0 f4 00 00 00 11
New:      c0 04 00 00 00 00
Kernel panic - not syncing: Corrupted kernel text
CPU: 0 PID: 9 Comm: migration/0 Not tainted 3.19.0-01975-gb1b096e70f23 #4
Call Trace:
  [<0000000000113972>] show_stack+0x72/0xf0
  [<000000000081f15e>] dump_stack+0x6e/0x90
  [<000000000081c4e8>] panic+0x108/0x2b0
  [<000000000081be64>] jump_label_bug.isra.2+0x104/0x108
  [<0000000000112176>] __jump_label_transform+0x9e/0xd0
  [<00000000001121e6>] __sm_arch_jump_label_transform+0x3e/0x50
  [<00000000001d1136>] multi_cpu_stop+0x12e/0x170
  [<00000000001d1472>] cpu_stopper_thread+0xb2/0x168
  [<000000000015d2ac>] smpboot_thread_fn+0x134/0x1b0
  [<0000000000158baa>] kthread+0x10a/0x110
  [<0000000000824a86>] kernel_thread_starter+0x6/0xc

Reported-and-tested-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>