Linux 5.16 compat: Check slab.h for kvmalloc

As it says on the tin - the folio work moved a bunch out of mm.h.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Coleman Kane <ckane@colemankane.org>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12975

config/kernel-kmem.m4

@@ -64,6 +64,7 @@ dnl #
 AC_DEFUN([ZFS_AC_KERNEL_SRC_KVMALLOC], [
 	ZFS_LINUX_TEST_SRC([kvmalloc], [
 		#include <linux/mm.h>
+		#include <linux/slab.h>
 	],[
 		void *p __attribute__ ((unused));