s390/setup: adjust start_code of init_mm to _text

After some investigation it doesn't look like init_mm fields
start_code/end_code are used anywhere besides potentially in dump_mm for
debugging purposes. Originally the value of 0 for start_code reflected
the presence of lowcore and early boot code. But with kaslr in place
start_code/end_code range should not span over unoccupied by the code
segment memory. So, adjust init_mm start_code to point at the beginning
of the code segment like other architectures do it.
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -1114,8 +1114,7 @@ void __init setup_arch(char **cmdline_p)
 
         ROOT_DEV = Root_RAM0;
 
-	/* Is init_mm really needed? */
-	init_mm.start_code = PAGE_OFFSET;
+	init_mm.start_code = (unsigned long) _text;
 	init_mm.end_code = (unsigned long) _etext;
 	init_mm.end_data = (unsigned long) _edata;
 	init_mm.brk = (unsigned long) _end;