uml: speed up exec

flush_thread doesn't need to do a full page table walk in order to clear the
address space.  It knows what the end result needs to be, so it can call unmap
directly.

This results in a 10-20% speedup in an exec from bash.
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/kernel/skas/exec.c

@@ -17,7 +17,17 @@
 
 void flush_thread_skas(void)
 {
-	force_flush_all();
+	void *data = NULL;
+	unsigned long end = proc_mm ? task_size : CONFIG_STUB_START;
+	int ret;
+
+	ret = unmap(&current->mm->context.skas.id, 0, end, 1, &data);
+	if(ret){
+		printk("flush_thread_skas - clearing address space failed, "
+		       "err = %d\n", ret);
+		force_sig(SIGKILL, current);
+	}
+
 	switch_mm_skas(&current->mm->context.skas.id);
 }