kernel/umh.c: optimize 'proc_cap_handler()'

If 'write' is 0, we can avoid a call to spin_lock/spin_unlock.

Link: http://lkml.kernel.org/r/20171020193331.7233-1-christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/umh.c

@@ -537,14 +537,14 @@ static int proc_cap_handler(struct ctl_table *table, int write,
 	/*
 	 * Drop everything not in the new_cap (but don't add things)
 	 */
-	spin_lock(&umh_sysctl_lock);
 	if (write) {
+		spin_lock(&umh_sysctl_lock);
 		if (table->data == CAP_BSET)
 			usermodehelper_bset = cap_intersect(usermodehelper_bset, new_cap);
 		if (table->data == CAP_PI)
 			usermodehelper_inheritable = cap_intersect(usermodehelper_inheritable, new_cap);
+		spin_unlock(&umh_sysctl_lock);
 	}
-	spin_unlock(&umh_sysctl_lock);
 
 	return 0;
 }