proc: enable writing to /proc/pid/mem

With recent changes there is no longer a security hazard with writing to
/proc/pid/mem.  Remove the #ifdef.

Signed-off-by: Stephen Wilson <[email protected]>
Signed-off-by: Al Viro <[email protected]>

fs/proc/base.c

@@ -854,10 +854,6 @@ out_no_task:
 	return ret;
 }
 
-#define mem_write NULL
-
-#ifndef mem_write
-/* This is a security hazard */
 static ssize_t mem_write(struct file * file, const char __user *buf,
 			 size_t count, loff_t *ppos)
 {
@@ -914,7 +910,6 @@ out_task:
 out_no_task:
 	return copied;
 }
-#endif
 
 loff_t mem_lseek(struct file *file, loff_t offset, int orig)
 {