Linux 4.2 compat: follow_link() / put_link()

As of Linux 4.2 the kernel has completely retired the nameidata
structure.  One of the few remaining consumers of this interface
were the follow_link() and put_link() callbacks.

This patch adds the required checks to configure to detect the
interface change and updates the functions accordingly.  Migrating
to the simple_follow_link() interface was considered but was decided
against ironically due to the increased complexity.

It also should be noted that the kernel follow_link() and put_link()
interfaces changes several times after 4.1 and but before 4.2.  This
means there is a narrow range of kernel commits which never appear
in an official tag of the Linux kernel which ZoL will not build.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Issue #3596

config/kernel-lookup-nameidata.m4

@@ -2,7 +2,7 @@ dnl #
 dnl # 3.6 API change
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_LOOKUP_NAMEIDATA], [
-	AC_MSG_CHECKING([whether iops->lookup() takes struct nameidata])
+	AC_MSG_CHECKING([whether iops->lookup() passes nameidata])
 	ZFS_LINUX_TRY_COMPILE([
 		#include <linux/fs.h>
 
@@ -18,7 +18,7 @@ AC_DEFUN([ZFS_AC_KERNEL_LOOKUP_NAMEIDATA], [
 	],[
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_LOOKUP_NAMEIDATA, 1,
-		          [iops->lookup() operation takes nameidata])
+		          [iops->lookup() passes nameidata])
 	],[
 		AC_MSG_RESULT(no)
 	])