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.

include/linux/mm_compat.h

@@ -0,0 +1,22 @@
+#ifndef _SPL_MM_COMPAT_H
+#define _SPL_MM_COMPAT_H
+
+#include <linux/mm.h>
+
+/*
+ * Linux 2.6.31 API Change.
+ * Individual pages_{min,low,high} moved in to watermark array.
+ */
+#ifndef min_wmark_pages
+#define min_wmark_pages(z)	(z->pages_min)
+#endif
+
+#ifndef low_wmark_pages
+#define low_wmark_pages(z)	(z->pages_low)
+#endif
+
+#ifndef high_wmark_pages
+#define high_wmark_pages(z)	(z->pages_high)
+#endif
+
+#endif /* SPL_MM_COMPAT_H */