smaps: only define clear_refs for CONFIG_MMU

/proc/pid/clear_refs is only defined in the CONFIG_MMU case, so make sure we
don't have any references to clear_refs_smap() in generic procfs code.

Signed-off-by: David Rientjes <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/base.c

···
 	.write		= oom_adjust_write,
 };
 
+#ifdef CONFIG_MMU
 static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 				size_t count, loff_t *ppos)
 {
···
 static struct file_operations proc_clear_refs_operations = {
 	.write		= clear_refs_write,
 };
+#endif
 
 #ifdef CONFIG_AUDITSYSCALL
 #define TMPBUFLEN 21