[S390] Mark kernel text section read-only.

Set read-only flag in the page table entries for the kernel image text
section. This will catch all instruction caused corruptions withing the
text section.
Instruction replacement via kprobes still works, since it bypasses now
dynamic address translation.

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

arch/s390/kernel/vmlinux.lds.S

@@ -35,10 +35,11 @@ SECTIONS
 
 #ifdef CONFIG_SHARED_KERNEL
   . = ALIGN(1048576);		/* VM shared segments are 1MB aligned */
-
-  _eshared = .;			/* End of shareable data */
 #endif
 
+  . = ALIGN(4096);
+  _eshared = .;			/* End of shareable data */
+
   . = ALIGN(16);		/* Exception table */
   __start___ex_table = .;
   __ex_table : { *(__ex_table) }