uml: physical memory shouldn't include initial stack

The top of physical memory should be below the initial process stack, not the
top of the address space, at least for as long as the stack isn't known to the
kernel VM system and appropriately reserved.

Cc: "Christopher S. Aker" <caker@theshore.net>
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/um/kernel/um_arch.c

@@ -258,6 +258,7 @@ int __init linux_main(int argc, char **argv)
 {
 	unsigned long avail, diff;
 	unsigned long virtmem_size, max_physmem;
+	unsigned long stack;
 	unsigned int i;
 	int add;
 	char * mode;
@@ -348,7 +349,9 @@ int __init linux_main(int argc, char **argv)
 	}
 
 	virtmem_size = physmem_size;
-	avail = TASK_SIZE - start_vm;
+	stack = (unsigned long) argv;
+	stack &= ~(1024 * 1024 - 1);
+	avail = stack - start_vm;
 	if (physmem_size > avail)
 		virtmem_size = avail;
 	end_vm = start_vm + virtmem_size;