Linux 2.6.31 Compatibility Updates

SPL_AC_2ARGS_SET_FS_PWD macro updated to explicitly include
linux/fs_struct.h which was dropped from linux/sched.h.

min_wmark_pages, low_wmark_pages, high_wmark_pages macros
introduced in newer kernels.  For older kernels mm_compat.h
was introduced to define them as needed as direct mappings
to per zone min_pages, low_pages, max_pages.

config/spl-build.m4

@@ -1266,6 +1266,7 @@ AC_DEFUN([SPL_AC_2ARGS_SET_FS_PWD],
 	[AC_MSG_CHECKING([whether set_fs_pwd() wants 2 args])
 	SPL_LINUX_TRY_COMPILE([
 		#include <linux/sched.h>
+		#include <linux/fs_struct.h>
 	],[
 		set_fs_pwd(NULL, NULL);
 	],[