Revert "x86 get_unmapped_area(): use proper mmap base for bottom-up direction"

This reverts commit df54d6fa.

The commit isn't necessarily wrong, but because it recalculates the
random mmap_base every time, it seems to confuse user memory allocators
that expect contiguous mmap allocations even when the mmap address isn't
specified.

In particular, the MATLAB Java runtime seems to be unhappy. See

  https://bugzilla.kernel.org/show_bug.cgi?id=60774

So we'll want to apply the random offset only once, and Radu has a patch
for that.  Revert this older commit in order to apply the other one.
Reported-by: NJeff Shorey <shoreyjeff@gmail.com>
Cc: Radu Caragea <sinaelgl@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/sys_x86_64.c

@@ -101,7 +101,7 @@ static void find_start_end(unsigned long flags, unsigned long *begin,
 				*begin = new_begin;
 		}
 	} else {
-		*begin = mmap_legacy_base();
+		*begin = TASK_UNMAPPED_BASE;
 		*end = TASK_SIZE;
 	}
 }

arch/x86/mm/mmap.c

@@ -98,7 +98,7 @@ static unsigned long mmap_base(void)
  * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64
  * does, but not when emulating X86_32
  */
-unsigned long mmap_legacy_base(void)
+static unsigned long mmap_legacy_base(void)
 {
 	if (mmap_is_ia32())
 		return TASK_UNMAPPED_BASE;

include/linux/sched.h

@@ -314,7 +314,6 @@ struct nsproxy;
 struct user_namespace;
 
 #ifdef CONFIG_MMU
-extern unsigned long mmap_legacy_base(void);
 extern void arch_pick_mmap_layout(struct mm_struct *mm);
 extern unsigned long
 arch_get_unmapped_area(struct file *, unsigned long, unsigned long,