kernel: explicitly include required header files under kernel/

Following an experimental deletion of the unnecessary directive

 #include <linux/slab.h>

from the header file <linux/percpu.h>, these files under kernel/ were exposed
as needing to include one of <linux/slab.h> or <linux/gfp.h>, so explicit
includes were added where necessary.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 47894f9..33acc424 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -45,6 +45,7 @@
 #include <linux/byteorder/swabb.h>
 #include <linux/stat.h>
 #include <linux/srcu.h>
+#include <linux/slab.h>
 
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Paul E. McKenney <paulmck@us.ibm.com> and "