s390: revert ELF_ET_DYN_BASE base changes

This reverts commit a73dc5370e.

Reducing the base address for 31-bit PIE executables from
(STACK_TOP/3)*2 to 4MB broke several compat programs which
use -fpie to move the executable out of the lower 16MB.

Cc: <stable@vger.kernel.org> # 4.13+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/elf.h

@@ -194,13 +194,14 @@ struct arch_elf_state {
 #define CORE_DUMP_USE_REGSET
 #define ELF_EXEC_PAGESIZE	PAGE_SIZE
 
-/*
- * This is the base location for PIE (ET_DYN with INTERP) loads. On
- * 64-bit, this is raised to 4GB to leave the entire 32-bit address
- * space open for things that want to use the area for 32-bit pointers.
- */
-#define ELF_ET_DYN_BASE		(is_compat_task() ? 0x000400000UL : \
-						    0x100000000UL)
+/* This is the location that an ET_DYN program is loaded if exec'ed.  Typical
+   use of this is to invoke "./ld.so someprog" to test out a new version of
+   the loader.  We need to make sure that it is out of the way of the program
+   that it will "exec", and that there is sufficient room for the brk. 64-bit
+   tasks are aligned to 4GB. */
+#define ELF_ET_DYN_BASE (is_compat_task() ? \
+				(STACK_TOP / 3 * 2) : \
+				(STACK_TOP / 3 * 2) & ~((1UL << 32) - 1))
 
 /* This yields a mask that user programs can use to figure out what
    instruction set this CPU supports. */