Linux 2.6.32 compat, proc_handler() API change

As of linux-2.6.32 the 'struct file *filp' argument was dropped from
the proc_handle() prototype.  It was apparently unused _almost_
everywhere in the kernel and this was simply cleanup.

I've added a new SPL_AC_5ARGS_PROC_HANDLER autoconf check for this and
the proper compat macros to correctly define the prototypes and some
helper functions.  It's not pretty but API compat changes rarely are.

config/spl-build.m4

@@ -72,6 +72,7 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_CRED_STRUCT
 	SPL_AC_GROUPS_SEARCH
 	SPL_AC_PUT_TASK_STRUCT
+	SPL_AC_5ARGS_PROC_HANDLER
 ])
 
 AC_DEFUN([SPL_AC_MODULE_SYMVERS], [
@@ -1371,3 +1372,22 @@ AC_DEFUN([SPL_AC_PUT_TASK_STRUCT], [
 		[__put_task_struct() is available])],
 		[])
 ])
+
+dnl #
+dnl # 2.6.32 API change,
+dnl # Unused 'struct file *' removed from prototype.
+dnl #
+AC_DEFUN([SPL_AC_5ARGS_PROC_HANDLER], [
+	AC_MSG_CHECKING([whether proc_handler() wants 5 args])
+	SPL_LINUX_TRY_COMPILE([
+		#include <linux/sysctl.h>
+	],[
+		proc_dostring(NULL, 0, NULL, NULL, NULL);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_5ARGS_PROC_HANDLER, 1,
+		          [proc_handler() wants 5 args])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])