brk: make sys_brk() honor COMPAT_BRK when computing lower bound

Fix a regression introduced by

commit 4cc6028d4040f95cdb590a87db478b42b8be0508
Author: Jiri Kosina <jkosina@suse.cz>
Date:   Wed Feb 6 22:39:44 2008 +0100

    brk: check the lower bound properly

The check in sys_brk() on minimum value the brk might have must take
CONFIG_COMPAT_BRK setting into account.  When this option is turned on
(i.e.  we support ancient legacy binaries, e.g.  libc5-linked stuff), the
lower bound on brk value is mm->end_code, otherwise the brk start is
allowed to be arbitrarily shifted.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -245,10 +245,16 @@ asmlinkage unsigned long sys_brk(unsigned long brk)
 	unsigned long rlim, retval;
 	unsigned long newbrk, oldbrk;
 	struct mm_struct *mm = current->mm;
+	unsigned long min_brk;
 
 	down_write(&mm->mmap_sem);
 
-	if (brk < mm->start_brk)
+#ifdef CONFIG_COMPAT_BRK
+	min_brk = mm->end_code;
+#else
+	min_brk = mm->start_brk;
+#endif
+	if (brk < min_brk)
 		goto out;
 
 	/*